It has been a while since I serviced one of these, but the DS1 circuit should have an NIU at the MPOE where Telco hands off the DS1 to you, usually an 8 position modular jack(RJ48C or X) or a punch block, depending how the DS1 was ordered and installed. The NIU should have monitor jacks on it where you can monitor the circuit with DS1 test equipment like a T-Berd test set. Also in the XPE and the host PBX, the DTI module should have monitor jacks, if I recall correctly. So you can check for errors, timing, level, etc. including loopback for complete testing and isolation of the problem. If you have a set of NTPs for the system, you can find a lot of info in the System Installation & Maintenance Guide. Back in the Meridian days it consisted of two binders. You might want to inspect your DS1 cable and wiring between the XPE and NIU on the remote end, and between the PBX and NIU on the host end for damage or bad terminations. The PBX itself has a maintenance history file that can be displayed. You might want to review it for any clues to hardware problems or what is happening and the frequency.
Since the PBX is digital, connecting circuits to a remote XPE are digital, known as a DS1, or T1 circuit. Same type used to connect two PBXs together when using DTI or PRI modules.
Hope this helps!
....JIM....