Too much noise on the line will cause your modem to disconnect. Speed of data transmission and reliablity of data transmission are inversly porportional. Reliability suffers with higher speeds, causing modems to disconnect and forcing you to reconnect to resume data transfer. Modems running over analog lines have a theoretical upper limit of 33.6 Kbps for bi-directional data transfer. While these high baud rates can occasionally be achieved, in the real world a baud rate of about 21.6 Kbps seems to provide the best throughput.
Several factors affect the reliability and speed of a modem connection over analog lines.
To overcome these limitations, modem manufactures have created modems that transfer data in an aysymetric manner, meaning that modems upload data at 19.2 Kbps, but download the data at speeds approaching 56 Kbps. These modems can only achieve these types of speeds when their connection is made over digital telephone lines and to a compatable modem on the other end of that digital line. V.90 employs one bidreictional channel for sending and receiving data. In essence, V.90 isolates the analog component so it does not affect the transmission of data, and thus V.90 lets modems receive data at up to 56 Kbps even over analog lines, as long as their is a digital connection at one end. Soon, after the bugs are worked out, the world will be totally V.90 compliant, but in the meantime ask your ISP if they are V.90 compliant, and find out if your new modem is also V.90 compliant.