The wiring inside your house can also induce noise in the conneciton if the wires are worn through or damaged. You can try to prefer V.90 or limit the maximum connection rate. You can also try your dialup from our office or a friends house to see if the connection speed improves on a different phone system. Bellsouth can check your local lines. A small amount of static that you can't hear can cause a noticable drop in connection speed.
Modems can go bad.
A modem may receive a static shock, from anyone or a lightning storm, which causes carbonization between silicon traces, as more and more electrons pass through this high resistance path it eventually becomes a low resistance path and a short circuit which can cause modem connection problems or modem failure. Some low quality modems can 'stall' which means they re-negiotiate the speed when they should not. Everytime a modem re-negiotiates the speed your connection seems to freeze. Some low quality modems can try to connect faster than the phone line is capable of supporting. This can be a problem if your local telephone line quality improves or decreases.