Dialing into the same account from two different computers simultaneously will cause both sessions to be logged out immediately, and e-mail will be sent explaining how this is against our Acceptable Use Policy . There's a separate FAQ entry for this particular problem also. Make sure nobody else in your home picks up a phone on the same line while you're online. The noise from this will cause the modems to freeze for a few seconds, or possibly hang up completely. You may be in an area with noisy or strangely configured phone lines.
Some modems deal with line impairments better than others.
On noisy lines, the modems try to shift their connect speed upwards and downwards during the call, to try to get the best speed possible with the fewest errors. The shifting process itself introduces delays. One way to work around some of these impairments is to manually slow your modem down slightly, so that it's not pushing the line so hard. Go to Control Panel and pick Modems. Highlight your modem and pick Properties.