Have this problem with 2 HD7850's and a Cooler Master 620W PS, if I OC the cards one throttles becouse the PS can't deliver the power needed (~400W, PS has not gotten cool air for a year, so it kinda got "broken")
I advise you to take a look at your PS...
Thanks...PSU is 850W though and i'm well below that with 3 cards. Also this is on a PSU with a single 12V rail and the other cards are happy.