Your tank is seriously lacking stam, only 15,5k HP. Buy all the BoE epics you can. Tempered helm and boots will give you a great boost. I think I had 23k when I first started heroics, but that was with a warrior though. Good luck.

http://www.wowhead.com/?item=41387 Helm
http://www.wowhead.com/?item=41392 Boots

They are cheap now