i just had a similar problem not too long ago. try show controllers t1 and specifically look at 'slip' errors that are incrementing on each interval. we've done looptest with telco and they corrected the problem with clock config on their side. you should press them for checking their configurations.
my t1 before:
T1 0/1/1 is up. (Line Looped (inband CSU code)) (Line Loop by Remote)
Applique type is Channelized T1
Cablelength is long gain36 0db
Description: "Slot 1 Port 1 VWIC2-2MFT T1 Card"
Receiver has no alarms.
alarm-trigger is not set
Soaking time: 3, Clearance time: 10
AIS State:Clear LOS State:Clear LOF State:Clear
Version info Firmware: 20071011, FPGA: 13, spm_count = 0
Framing is ESF, Line Code is B8ZS, Clock Source is Line.
CRC Threshold is 320. Reported from firmware is 320.
Data in current interval (714 seconds elapsed):
0 Line Code Violations, 1182 Path Code Violations
715 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 11 Degraded Mins
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