Some inexpensive modems have poor isolation between the load of the modem and the load of the telephone instrument, and this can cause high-speed problems as well. If you're having problems connecting with a certain BBS on a certain exchange, try another BBS on another exchange. The interconnect from your local exchange may pass through many other exchanges, some of which may have problems interconnecting to still other exchanges. Isolating the problem to a certain part of town won't make things any better, but you'll know the problem isn't in your house wiring or your modem. This also works for long-distance calls as well.
Some area codes are notorious for bad connections, while others work well.
Also, depending on your long distance provider, you may experience problems only with long distance and nothing else. Early on, I had a similar problem with one provider and switched to another, which solved my problem. Sad to say, that some modem manufacturers do a poor job of implementing the V.34 standard. To figure out your problem, start at the basics. You need a DTE speed greater than your highest connect speed to have good throughput, and with some modems, if the DTE rate is lower than the connect speed, the modem will fallback to a lower speed.