We lost one T1 here a while ago. We had just replaced the CSU/DSU monday after one failed over the weekend. I called a ticket into Sprint but afterwards I thought, well, I had better roll the circuits to make sure that our new CSU/DSU hadn't failed. I rolled the circuits and the trouble stayed with the CSU; I then rolled router ports to determine if it was the CSU/DSU or the router port. When I did, both circuits came up. I restored to the original configuration, both circuits remained up. At this point I don't know if it's CSU, router port, or flakey v.35 cable connection since everything came back up after the roll. If it turns out to be a problem with the 3-port card in the router, I have a spare 4-port card I can replace it with. But at this point we have to catch it dead again before we can determine what is wrong.