I know this may sound pretty basic but we have a lot of PRI's here and usually up down symptoms (if not a problem from the CO) are due to cabling.
Check your connection from the PRI Interface and your DTM card.
Obviously, the shorter the distance the better.
Insure it is on proper cabling (No flat cabling..)
Away from EMI source.
Check RJ45 carefully. make sure there is no strain on the connector.
Check Cabling from Telco NID to PRI jack.
We have had up/down problems that were due to,
1) Coils and excessive length in the Cable from the NID to the Jack.
2) A RJ45 connect or solid copper CAT5 cable that would test OK but have a lose connection once a bit of strain was on it.
3) Someone installed Silver Satin on the PRI - (I'm surprised it came up in the first place)
4) the PRI Cable running by a compressor motor that induced voltage when it started.
The best way is to test for loss and channel up status - especially with the CO so they can tell you if they see it going up and down.
What you describe sounds like it keeps going up and down at times.
Again, I know this is all basic, but sometimes the simplest solutions are the correct ones.