I think you probably have answered your own question, in your original post. When you discussed the "spiky" damage, that set off an alarm for me, that possibly it could be the types of heals that are incoming. Perhaps a discussion with your healers would illuminate. As a warrior tank, I tend to like the sweet Hots of druidic goodness, especially just before a charge, when my rage has not kicked in yet (sorry, I don't know names of spells from various other classes--hopefully, you get the idea, though). A continuous flow of healing, like a totemic presence, is also wonderful...a good base from which to keep me topped-off. I'm told that Hunter misdirects are helpful, too, in some cases...also, you might try to remind the Warr to use his Vigilance--if he's pug, then let him know who has the most deeps, or who is most likely to "jump the gun" on the dps meter.
As for threat, only time I have trouble with that is if not given a few seconds to start--generally I call for at least a "count to six" before unleashing damage...or, I just simply say, "Watch for me to jump up and down, before you pew-pew!"
Bear in mind, I'm not a raiding tank, yet, other than OS, and a bit of NAXX. However, I think if you keep these couple of things in mind, it should fare better. Barring just having a bad tank, or a good tank, on a bad night, I don't think there's anything all that wrong with the class doing the job.
As for your question about pally spike damage...I'm not sure, but I've OT'd with a pally tank quite a bit, and he tends to talk to himself, rather harshly, when he forgets to cast certain spells, before a pull...maybe that's some kind of mitigation to the spike damage?
-"Just like a buzzin' fly, I come into your life, I'll float away, like honey in the sun..."--Tim Buckley