Does your system allow you to clear the counters for the bipolar (line coding) violations? If so, reset the counters, and monitor how rapidly they are accumulating. If they are accumulating very rapidly, see if XO will send someone out to check their smartjack (tell them they need to check for a "dirty jack").
I've encountered bipolar errors in the past, similar to what you're experiencing, but the errors were accumulating very rapidly. It turned out to be caused by a faulty connection between a PRI card and it's smartjack/chasis. The card was simply reseated, we cleared the counters again, and no bipolar errors have been recorded eversince. The provider referred to this type of cause as "a dirty jack".
At least see if you can clear the counters to see how rapidly they are accumulating. Since this symptom is intermittent, see if the customer can provide any types of patterns whatsoever which may help out in narrowing this down.
