I my experience this is usually a Telco fault.

What I would do is perform a Digitrace and leave it running to monitor then get the time when the fault occurs and look back at the trace file.
You will see Rx errors and with this information/proof you will be able to report the fault to the Telco provider.
As you said when the PRI is unplugged the system works as normal.
You could always swap out the BPTU card if you have a spare to eliminate this as a possible cause (but I doubt it is the issue).

Regards
Carl