WAN troubleshooting wth SQL database

I am running an SQL 2000 database for remote users over a T1. The users are complaining of periodic lockups and inability to log in. When we look at the T1, it has had periodic bounces but no consistent problem. At the time that the users can’t log in, no one can log in from any location so it seems like the problem is the T1. I need some help on where to start with the troubleshooting.