Document:

<PAGE>   1
                                                                    EXHIBIT 10.6

                              AMENDED AND RESTATED
                            TAX" ALLOCATION AGREEMENT
                                SUPPLEMENT NO. I

This Supplement, dated as of the 31st day of August, 1999, is by and among MBIA
Inc., a Connecticut stock insurance company ("P"), MBIA Insurance Corporation
("Corp."), a New York corporation, and a subsidiary of Corp., MBIA Insurance
Corp, of Illinois ("MBIA Illinois"), an Illinois corporation.

WITNESSETH:

WHEREAS, P and Corp. originally entered into a Tax Allocation Agreement (the
"Agreement") dated as of January 1, 1987, subsequently amended and restated as
of January 1, 1990, attached hereto as Exhibit A; and

WHEREAS, the fourth WHEREAS clause in the Agreement extends the Agreement to
Corp.'s present and future subsidiaries; and

WHEREAS, MBIA Illinois is a wholly-owned subsidiary of Corp.; and

WHEREAS, MBIA Illinois is required to be in compliance with all applicable
Illinois laws and regulations and therefore must add certain provisions to the
Agreement;

NOW, THEREFORE, IT IS AGREED AS FOLLOWS:

1.  Pursuant to 50 Ill. Adm. Code 942, and only with respect to MBIA Illinois:

Settlements will be no later than thirty (30) days after the filing of the
return, unless a refund is due from a taxing authority, in which case the
settlement may be thirty (30) days after receipt of the refund from the taxing
authority.

2. Pursuant to 50 Ill. Adm. Code 942. and only with respect to MBIA Illinois:

All settlements must be in the form of cash or securities eligible for
investment under the Illinois Insurance Code.

IN WITNESS WHEREOF, the parties have caused this Supplement to be executed by
their duly authorized officers, effective as of August 31st 1999.

MBIA INC.

By:      Gary C. Dunton President

MBIA INSURANCE CORPORATION

By:      Gary c. Dunton, President

MBIA INSURANCE CORP. OF ILLINOIS

By:      Richard L. Weill, President<PAGE>   1
                                SECOND AMENDMENT

                                       to

                  SECOND AMENDED AND RESTATED CREDIT AGREEMENT

                                      among

                           MBIA INSURANCE CORPORATION
                                     (MBIA)

                           THE BANKS SIGNATORY HERETO

                               RABOBANK NEDERLAND
                                 New York Branch
                             as Administrative Agent

                                       and

                                DEUTSCHE BANK AG
                                 New York Branch
                             as Documentation Agent

                          Dated as of October 29, 1999
<PAGE>   2
                                SECOND AMENDMENT

THIS SECOND AMENDMENT, dated as of October 29,1999 (this "Amendment"), between
MBIA INSURANCE CORPORATION, a New York stock insurance corporation ("MBIA"), the
financial institutions which have executed this Amendment below as Banks (as
defined below), COOPERATIEVE CENTRALE RAIFFEISEN-BOERENLEENBANK B.A. "RABOBANK
NEDERLAND", New York Branch ("Rabobank"), as Administrative Agent for the Banks
(in such capacity, the "Administrative Agent") and individually as a Bank, and
DEUTSCHE BANK AG, New York Branch, as Documentation Agent for the Banks (in such
capacity, together with the Administrative Agent, the "Agents") and individually
as a Bank;

WHEREAS, the parties hereto are parties to the Second Amended and Restated
Credit Agreement, dated as of October 1, 1997, as amended by the First Amendment
thereto dated as of October 1, 1998 and as further modified by certain
Assignment and Assumption Agreements (as defined therein) (as so amended and
modified, the "Credit Agreement"); and

WHEREAS, the parties hereto desire, upon the terms and subject to the conditions
hereinafter set forth, to extend the Expiration Date (as defined below) and to
otherwise modify the Credit Agreement in certain respects;

NOW, THEREFORE, in consideration of the mutual promises contained herein and
other valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereto, intending to be legally bound hereby, agree as
follows:

                                    ARTICLE I

                         MODIFICATIONS TO LOAN DOCUMENTS

Section 1. 1. Defined Terms. Except as otherwise specified herein, terms used in
this Amendment and defined in Exhibit A of the Credit Agreement shall have the
meanings provided in such Exhibit A.

Section 1.2. Amendment. (a) The definition of the term "Expiration Date"
contained in Exhibit A to the Credit Agreement is hereby amended and restated to
read in its entirety as follows:

"'Expiration Date' shall mean the date on which the right to obtain Loans
terminates, initially October 31, 2006, as such date may be extended pursuant to
Section 3.3."

(b) The definition of the term "Loan Commencement Event" contained in Exhibit A
to the Credit Agreement is hereby amended and restated to read in its entirety
as follows:

"'Loan Commencement Event' shall mean the time at which Cumulative Losses for
the current Commitment Period first exceed the aggregate Pledged Recoveries
received by MBIA during the current Commitment Period by an amount equal to the
greater of (a) the highest Maximum Commitment at any time during the such
Commitment Period and (b) four percent (4%) of Average Annual Debt Service on
the Covered Portfolio."

Section 1.3 Commitments. The aggregate Commitments of the Banks are hereby
increased pursuant to Section 10.8(d) of the Credit Agreement and further
amended so that, from and after October 29, 1999 until the termination or
further modification thereof as provided in the Credit Agreement, such
Commitments shall be as set forth on Schedule I to this Amendment.

Section 1.4 Amendment to Security Agreement Each Bank and the Administrative
Agent hereby consent to an amendment to the Security Agreement which will
provide that the definition of the term "Special Event of Default" contained
therein be amended and restated to read in its entirety as follows:

"'Special Event of Default' means any of (i) any Event of Default described in
clause (i) of paragraph (a) of Section 7.1 of the Credit Agreement in respect of
principal of or interest on the Loans or the Notes, (ii) any MBIA Event of
Insolvency, or (iii) if MBIA shall have incurred Cumulative Losses during the
current Commitment Period of more than the greater of (A) 50% of the highest
Maximum Commitment at any time during the current Commitment Period (or, if the
last Commitment Period has expired, at any time during such Commitment Period),
and (B) 2% of Average Annual Debt Service on the Covered Portfolio, any failure
of MBIA to perform or observe the covenant contained in Section 6.8 of the
Credit Agreement."

                                    ARTICLE 2

                              CONDITIONS PRECEDENT

 .Section 2.1. Conditions Precedent to Amendment Effective Date. The provisions
of Article I hereof shall become effective as of October 29, 1999 when this
Amendment shall have been executed and delivered by MBIA, each Agent and
consented to by each Bank and when the following conditions have been fulfilled
to the reasonable satisfaction of the Agents. If such conditions shall not have
been satisfied on or prior to November 19, 1999, the provisions of Article I
shall not be given effect unless otherwise consented to by the Agents and the
Majority Banks, but otherwise this Amendment shall remain in full force and
effect.

(a) There shall exist no Default or Event of Default, and all representations
and warranties made by MBIA herein or in any of the Loan Documents shall be true
and correct with the same effect as though such representations and warranties
had been made at and as of such time.

(b) The Administrative Agent shall have received each of the following, in form
and substance satisfactory to the Administrative Agent:
<PAGE>   3
(i) a certificate of any two of the President, Vice Chairman, Managing Director,
any Vice President or the Treasurer of MBIA to the effect that the conditions
set forth in Section 2. 1 (a) hereof have been satisfied and that no
governmental filings, consents and approvals are necessary to be secured by MBIA
in order to permit the borrowing under the Credit Agreement, as modified hereby,
the grant of the Lien under the Security Agreement and the execution, delivery
and performance in accordance with their respective terms of this Amendment and
the other Loan Documents and the consummation of the transactions contemplated
hereby and thereby, each of which shall be in full force and effect;

(ii) copies of the duly adopted resolutions of the Board of Directors of MBIA,
or an authorized committee thereof, authorizing the execution, delivery and
performance in accordance with their respective terms of this Amendment and the
other documents to be executed and delivered by MBIA described herein
(collectively, the "Amendment Documents"), accompanied by a certificate of the
Secretary or an Assistant Secretary of MBIA stating as to (A) the effect that
such resolutions are in full force and effect, (B) the incumbency and signatures
of the officers signing the Amendment Documents on behalf of MBIA, and (C) the
effect that, from and after October 1, 1998, there has been no amendment,
modification or revocation of the articles of incorporation or by-laws of MBIA;

(iii) opinions of the General Counsel of MBIA and Kutak Rock, MBIA's counsel,
each dated October 29, 1999, which are substantially to the effect set forth in
the forms attached hereto as, respectively, Exhibits A and B; and

(iv) such other documents, instruments, approvals (and, if reasonably requested
by the Administrative Agent or the Majority Banks, duplicates or executed copies
thereof certified by an appropriate governmental official or an authorized
officer of MBIA) or opinions as the Administrative Agent or the Majority Banks
may reasonably request.

(c) The Administrative Agent shall have received reasonably satisfactory
evidence that long-term obligations insured by MBIA are publicly assigned a
rating of Aaa by Moody's and AAA by S&P by reason of such insurance.

(d) The Bank Fee Letter shall have been modified in a manner satisfactory to
MBIA and the Agents and consented to by all of the Banks.

(e) Each Bank which is becoming a party to the Credit Agreement or which is
increasing its Commitment shall have received a Note or an additional Note dated
as of October 29, 1999, in a principal amount equal to the amount of its
Commitment or of the increase in its Commitment, as applicable.

(f) The Security Agreement shall have been amended as contemplated by Section
1.4 hereof.

(g) The currently effective Fronting Bank Supplements and related Fronting Bank
Notes and fee letters shall have been modified in a manner satisfactory to MBIA,
the Administrative Agent and each Fronting Bank affected by such modifications.

(h) All corporate and legal proceedings and all instruments in connection with
the transactions contemplated by this Amendment and the Loan Documents shall be
satisfactory in form and substance to the Administrative Agent and its counsel.

Section 2.2. Certificate as to Effective Date. A certificate of the Agents
delivered to MBIA stating that the provisions of Article I shall have become
effective shall be conclusive evidence thereof and shall be binding on MBIA,
each Agent and each Bank. In delivering such certificate, and without limiting
the general application of Section 8.8 or other provisions of Article 8 of the
Credit Agreement to the actions of the Agents hereunder, the Agents shall be
entitled to rely conclusively on the certificate of officers of MBIA delivered
pursuant to Section 2.1(b)(i) as to the satisfaction of the conditions set forth
in Section 2.1(a).

                                    ARTICLE 3

                         REPRESENTATIONS AND WARRANTIES

In order to induce the Agents and the Banks to enter into this Amendment and
proceed with the transaction contemplated hereby, MBIA makes the following
representations and warranties to the Agents and the Banks, which shall survive
the execution and delivery of this Amendment and the making of any Loans:
<PAGE>   4
Section 3.1. Due Authorization, Etc. The execution, delivery and performance by
MBIA of the Amendment Documents and the Loan Documents as amended thereby are
within its corporate powers, have been duly authorized by all necessary
corporate action and do not and will not (i) violate any provision of any law,
rule, regulation (including, without limitation, the New York Insurance Law, the
Investment Company Act of 1940, as amended, or Regulations T, U or X of the
Board of Governors of the Federal Reserve System), order, writ, judgment,
injunction, decree, determination or award presently in effect having
applicability to MBIA or of the corporate charter or by-laws of MBIA, (ii)
result in a breach of or constitute a default under any indenture or loan or
credit agreement or any other agreement, lease or instrument to which MBIA is a
party or by which it or its properties may be bound or affected, or (iii) result
in, or require, the creation or imposition of any Lien upon or with respect to
any of the properties now owned or hereafter acquired by MBIA (other than as
contemplated by the Loan Documents), other than, in the case of clauses (ii) and
(iii), breaches, defaults or Liens which could not materially and adversely
affect the business, assets, operations or financial condition of MBIA or the
ability of MBIA to perform its obligations under any Loan Document.

Section 3.2. Approvals. No consent, approval or other action by, or any notice
to or filing with any court or administrative or governmental body is or will be
necessary for the valid execution, delivery or performance by MBIA of the
Amendment Documents or the Loan Documents as amended thereby.

Section 3.3. Enforceability. Each Amendment Document and each Loan Document as
amended thereby constitutes a legal, valid and binding obligation of MBIA,
enforceable against MBIA in accordance with their respective terms, except as
such enforceability may be limited by bankruptcy, insolvency, moratorium or
other similar laws affecting the enforcement of creditors' rights generally and
the availability of equitable remedies, whether such matter is heard in a court
of law or a court of equity.

Section 3.4. Financial Statements, etc. (a) MBIA has heretofore furnished to the
Agents (1) the audited consolidated and unaudited consolidating balance sheets
of MBIA Inc. and its subsidiaries at December 31, 1998, the related audited
consolidated statements of income, changes in stockholders' equity and financial
position or cash flows, as the case may be, and unaudited consolidating
statements of income for the year ended December 31, 1998, and (ii) the
unaudited consolidated and consolidating balance sheets of MBIA Inc. and its
subsidiaries as of March 31 and June 30, 1999, and the related consolidated
statements of income, changes in stockholders' equity and cash flows for the
three months ended March 31, 1999, the six months ended June 30, 1999. Such
financial statements were prepared in accordance with generally accepted
accounting principles consistently applied and present fairly the consolidated
financial position and consolidated results of operations and cash flows of MBIA
Inc. and its subsidiaries and the financial position and results of operations
and cash flows of MBIA at the dates and for the periods indicated therein. There
has been no material adverse change in the consolidated financial position or
consolidated results of operations or cash flows of MBIA Inc. and its
subsidiaries taken as a whole or of MBIA since June 30, 1999.

(b) MBIA has heretofore furnished to the Agents its annual statements and its
financial statements as filed with the Department for the year ended December
31, 1998 and its quarterly statements and financial statements as filed with the
Department for the periods ended March 31, 1999 and June 30, 1999. Such annual
and quarterly statements and financial statements were prepared in accordance
with the statutory accounting principles set forth in the New York Insurance
Law, all of the assets described therein were the absolute property of MBIA at
the dates set forth therein, free and clear of any liens or claims thereon,
except as therein stated, and each such Annual Statement is a full and true
statement of all the assets and liabilities and of the condition and affairs of
MBIA as of such dates and of its income and deductions therefrom for the year or
quarter ended on such dates.

(c) MBIA has heretofore furnished to the Agents a copy of the annual report on
Form 10-K of MBIA Inc. for the fiscal year ended December 31, 1998, its
quarterly reports on Form 10-Q of MBIA Inc. for each of the quarters ended March
31, 1999 and June 30, 1999 and each current report on Form 8-K filed by MBIA
Inc. on or after January 1, 1999, each as filed with the Securities and Exchange
Commission. Such annual, quarterly and current reports were prepared in
accordance with the Securities Exchange Act of 1934, as amended, and the rules
and regulations promulgated thereunder.

Section 3.5. Covered Portfolio. Substantially all of the Insured Obligations in
the ,Covered Portfolio are insured by MBIA under Insurance Contracts in the form
or forms heretofore supplied to the Agents in accordance with MBIA's
underwriting criteria as heretofore disclosed to the Agents, and in MBIA's
reasonable judgment such Insured Obligations represent an overall risk of loss
(based on all factors including without limitation investment quality and
geographical and market diversification) which is not materially greater than
the risk of loss represented by all of MBIA's Insured Obligations as of the date
hereof.

Section 3.6. Confirmation of Representations and Warranties. MBIA hereby
confirms that its representations and warranties set forth in the Credit
Agreement (including without limitation those set forth in Article 5 of the
Restated Credit Agreement) are true and correct as of the date hereof.

Section 3.7. Disclosure. There is no fact known to MBIA which materially
adversely affects the business, assets, operations or financial condition of
MBIA or the ability of MBIA to perform its obligations under any Amendment
Document or any Loan Document as amended thereby which has not been set forth in
this Amendment, in the financial statements or reports required to be delivered
pursuant to Section 3.4 hereof

                                    ARTICLE 4

                                  MISCELLANEOUS

Section 4.1. Credit Agreement. Except as expressly modified as contemplated
hereby, the Credit Agreement and the other Loan Documents are hereby confirmed
to be in full force and effect in accordance with their respective terms. This
Amendment is intended by the parties to constitute an amendment and modification
to, and otherwise to constitute a continuation of, the Credit Agreement and the
Loan Documents, and is not intended by any party and shall not be construed to
constitute a novation thereof or of any Debt of MBIA hereunder.
<PAGE>   5
Section 4.2. Survival. All covenants, agreements, representations and warranties
made herein or in any Loan Document or in any certificate, document or
instrument delivered pursuant hereto or thereto shall survive the effective date
hereof, the making of any Loan and the occurrence of the Expiration Date and
shall continue in full force and effect so long as principal of or interest on
any Loan, Note or Fronting Bank Note remains outstanding or unpaid, any other
amount payable by MBIA under the Credit Agreement as amended hereby, any Note,
Fronting Bank Note or any other Loan Document remains unpaid or any other
obligation of MBIA to perform any other act hereunder or under the Credit
Agreement as amended hereby, any Note, Fronting Bank Note or any other Loan
Document remains unsatisfied or the Banks have any obligation to make a Loan or
any other advance of moneys to MBIA under the Credit Agreement as amended
hereby.

Section 4.3. Severability Any provision of this Amendment which is prohibited,
unenforceable or not authorized in any jurisdiction shall, as to such
jurisdiction, be ineffective to the extent of such prohibition, unenforceability
or nonauthorization without invalidating the remaining provisions hereof or
affecting the validity, enforceability or legality of such provision in any
other jurisdiction.

Section 4.4. Successors and Assigns. This Amendment is a continuing obligation
and binds, and the benefits hereof shall inure to, the parties hereto and their
respective successors and assigns; provided that MBIA may not transfer or assign
any or all of its rights or obligations hereunder except as permitted by Section
10.8 of the Credit Agreement.

Section 4.5. Amendments. No provision of this Amendment shall be waived, amended
or supplemented except as provided in Section 10.12 of the Credit Agreement.

Section 4.6. Governing Law. THIS AMENDMENT SHALL BE GOVERNED BY, AND CONSTRUED
IN ACCORDANCE WITH, THE LAW OF THE STATE OF NEW YORK.

Section 4.7. Headings. Section headings in this Amendment are included herein
for convenience or reference only and shall not constitute a part of this
Amendment for any other purpose.

Section 4.8. Counterparts. This Amendment may be executed in several
counterparts, each of which shall be regarded as the original and all of which
shall constitute one and the same Amendment.

IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be duly
executed and delivered by their respective officers thereunto duly authorized as
of the date first above written.

MBIA INSURANCE CORPORATION

By

Name: Neil G. Budnick
Title: Vice Chairman and
       Chief Financial Officer
<PAGE>   6
COOPERATIEVE CENTRALE RAIFFEISEN-BOERENLEENBANK B.A. "RABOBANK NEDERLAND", New
York Branch, as Administrative Agent and as a Bank

By

Name:    Angela R. Reilly
Title:   Vice President

By

Name:    Robert S. Bucklin
Title:   Chief Corporate Banking Officer

DEUTSCHE BANK AG, New York Branch, as Documentation Agent and as a Bank
By

Name:    John S. McGill
Title:   Director

By

Name:    Clinton M. Johnson
Title:   Managing Director
<PAGE>   7
SCHEDULE I
TO SECOND AMENDMENT

                        BANKS. ADDRESSES AND COMMITMENTS

<TABLE>
<CAPTION>
Name and Notice Address of Bank                                         Commitment
<S>                                                                   <C>
Banco Santander Central Hispano S.A.,                                 $ 20,000,000
  New York Branch
45 East 53rd Street
New York, NY 10022
Attn: Shailesh Deshpande

Bank of America, NA                                                   $ 65,000,000
231 South LaSalle Street
  Chicago, IL 60697
Attn: Joan D'Amico

The Bank of New York                                                  $ 40,000,000
Insurance Division
1 Wall Street, 17th Floor
New York, NY 10286
Attn: Louis DiFranco

The Bank of Nova Scotia                                               $ 25,000,000
One Liberty Plaza
New York, NY 10006
Attn: Brian Allen

Bayerische Landesbank Girozentrale,                                   $ 50,000,000
  New York Branch
560 Lexington Avenue, 17th Floor
New York, NY 10022
Attn: Scott Allison

CDC Financial Products, Inc.                                            50,000,000
9 West 57th Street - 36th Floor
New York, NY 10019
Attn: David L. Askren, Senior VP

The Chase Manhattan Bank                                              $ 25,000,000
270 Park Avenue, 20th Floor
New York, NY 10017
Attn: Robert Foster

Commonwealth Bank of Australia,                                       $ 25,000,000
  New York Branch
599 Lexington Avenue
New York, NY 10022
Attn: Patrick Hildreth

Credit Local de France, New York Agency                               $ 10,000,000
450 Park Avenue, 3rd Floor
New York, NY 10022
Attn: James Beck

Credit Suisse First Boston, New York Branch                           $100,000,000
Eleven Madison Avenue
New York, NY 10010-3629
Attn: James Lee

Deutsche Bank AG, New York Branch                                     $100,000,000
Deutsche Bank Securities Inc.
31 West 52nd Street
New York, NY 10019
Attn: John McGill

DGZoDekaBank Deutsche Kommunalbank                                    $ 25,000,000
International Finance Department
Taunusanlage 10
D-60329 Frankfurt am Main
GERMANY
Attn: Stephan Wagner
</TABLE>
<PAGE>   8
<TABLE>
<CAPTION>
Name and Notice Address of Bank                                         Commitment
<S>                                                                   <C>
Bank One, NA                                                          $ 10,000,000
153 West 51st Street
New York, NY 10019
Attn: Timothy Stambaugh

Fleet National Bank                                                   $ 40,000,000
777 Main Street, CT-MO 0250
Hartford, CT 06115
Attn: Elizabeth Shelley, VP

KBC Bank, N.V.                                                        $ 20,000,000
  125 West 55th Street
New York, NY 100 19
Attn: Patrick Owens

Landesbank Hessen-Thuringen Girozentrale,                             $ 50,000,000
  New York Branch
420 Fifth Avenue, 24th Floor
New York, NY 10018
Attn: John Sarno

Lloyds TSB Bank Plc                                                   $ 50,000,000
575 Fifth Avenue, 18th Floor
New York, NY 10017
Attn: Peter Kemick

Norddeutsche Landesbank Girozentrale,                                 $ 20,000,000
  New York Branch
1270 Avenue of the Americas, 14th Floor
New York, NY 10020
Attn: Stephanie Finneran

Westdeutsche Landesbank Girozentrale                                  $ 50,000,000
  New York Branch
1211 Avenue of the Americas
New York, NY 10036
Attn: Lillian Tung Lum, VP

Cooperatieve Centrale Raiffeisen-Boerenleenbank B.A.                  $125,000,000
"Rabobank Nederland", New York Branch
245 Park Avenue, 37th Floor
New York, NY 10167
Attn: Angela R. Reilly
                                                 TOTAL                $900,000,000
</TABLE>

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