So, you went out and bought a new 28.8 modem only to discover that it can't hold a connection at 28.8. Don't return it just yet. It may be your configuration.
Check your user's manual to see if you can change the maximum frame size on your modem. If you can change the maximum frame size, start with the maximum and then work your way towards the minimum until you find a size that will work. (On the AT&T DataPort 28.8 modem, for example, the command is "AT/A1" to set the maximum frame size to 28 characters per frame.)
You may also need to experiment with the modulation rate. Although modems negotiate the modulation rate and attempt to connect at the highest possible rate, this doesn't always work. Sometimes you must fix the maximum rate to 14.4, 16.8, 19.2, 24.0, etc.
Experimenting with both of these parameters may take a while, but it will be worth the time spent--especially if you are making long-distance calls to transfer files.