If all of you T1s/PRI circuits are getting slipps then it might be the loop with the clocking or the clock controller. If only 1 or 2 are getting it, it might be the cards the cicuits are on,
and the last one .... it could be the TELCO....
TBERD is the best option like PERRYPJ said, you can "see the inbound and outbound" to determine if it's your equipment or the CO....
it sounds like you have a DDP card from your post, if you are getting slipps on that DDP and only that one and no other loops then you migth want to reseat (disable properly first) that card during a maintenance window.