Well, every single possible solution to fix your problem were stated above, but since it still the same problem, do like i do in such situations and think of something irrelevant
, could you please try a different PCIE slot ? i know it has nothing to do with it but give it a try, and also clean the PCI slots from dust if there any.....
Also borrow any GPU ( Preferred the same model or equivalent ) from your friends and try,,,if every thing goes well, then it is a hardware issue...if not...then it is something either about your motherboard or its drivers........