After re-reading your original post I've a few questions. Does this terminate into an AT&T switch? I'm assuming it does. Does the circuit run over the AT&T (old AT&T) network? If it does there is monitoring equipment on it 24/7 and they can get in at any time without breaking the circuit and see what's going on. Is there still an external CSU on this? And is it optioned correctly? I'm a little bit leery about the slip thing for the reason mentioned above. If it truly is slipping then it can only be your equipment causing it. I doubt it would work flawlessly for 7 months if there were originally an option problem. Next time this starts acting up, unplug from the demarc for a few seconds and plug it back in. If that cures the problem you definitely have a timing issue.

Retired phone dude