I had a similar problem with an 11C where random digital tn's would just quit working. After checking the power, grounding, and re-seating the same 3-4 cards, the problem strangely enough turned out to be the customer supplied T1.
Turns out they were emulating a T1 through thier network, and there was some conversion issue that was causing the grief. I was new to the site/customer and had no idea how the T1 was supplied.
Doubt this will help you, but I thought I'd share it.