Patch cable, extended dmarc, distance from telco NIU, bad cable pairs, CSU issue, etc. Did you double check these? Did you have telco perform a stress test on the circuit? There are just so many variables and you said that the problem is not consistent. You might want to have your vendor work with Toshiba tech support. There are software tools available from Toshiba in addition to trace files to help your vendor look in the right direction.