If you have a modem that supports v. FC but not v.34 for 28.8 connections, that may be the cause of your problem. You can get this information out of your modem's manual. Most modem manufacturers offer an upgrade that will allow you to upgrade your v. FC modem to a v.
Inactivity can also cause disconnections.
Net has a 25 minute inactivity time-out, meaning that if you do not send any data over your Internet connection for 25 minute, your account will be disconnected. Most Internet dialer software will automatically drop the network connection after a few minutes of inactivity. Minimize the window, and your connection will hold up. Some modems are better than others at holding connections. If that is not the case, consult your modem's manual for details.