You could try adding the secret cisco command to eliminate all problems on your router=: router>#bugs off
Only kidding ;-), there is no such command, though there are some that would have tried it.... Your problem could be caused by many things so its hard for anyone to assist without you first doing some initial troubleshooting and recording the results.
1. It could be your connection. I gather you have digital modems connecting through a T1. Check with your line provider to see if they have experienced problems with the T1 line. They should be able to produce logs of the line up-time.
2. You should try connecting as a remote user and see if you can simulate the error to confirm the exact nature of the problem.
3. Check the log on your router.
If you post your config, we may be able to see if there is something configured incorrectly.
JimmyZ