I can't help you with the DK stuff since I'm from the light side as pally but maybe have a different idea for you.
Try talking to the healers. Happened to another tank at my side recently that he was dropping like a rock but doing fine. After chatting a little with the healers on ts after the raid it came out that this guy was really (and I mean REALLY) unpopular with the healing staff. They were kind of unmotivated to elongate his lifespan.
Maybe it's not even that but a kind of tactic thing. If the heals are familiar with the regular tanks and they always use cds at certain points and the new tanks don't... well.
That's propable not true in your case but as it happened to me lately I just had to remember and write...