Although this would seem to be a hardware problem, in one case it was narrowed down to an extension conflict. Go to a Mac OS base set plus all Digidesign, Open Music System, SVI.cf, and any other extensions absolutely necessary for Pro Tools to run (i.e. iLok, Waves, USB Floppy Enabler, etc.)In one case, tests failed even after all Avid hardware was swapped. Loopback barrels were connected while running the test.