I have been having this problems on and off after a few months of owning my computer
. I thought it was my power supply at first simply because it was rather crappy but that has now been replaced now
then I thought it was the gpu that was going bad but that does not seem to be the case from what i can tell by swaping them around in every way i can think of. I have swaped the power connecters going to them as well as swaping there positions.
I have also gotten a new motherboard and reinstalled my OS and upgraded to 8 as well, though not in an attempt to fix the issue.
I have taken apart this computer and looked at the components many time I have taken apart the gpu a few times too but not in a while but never noticed a problem. tho it is possible i was reckless and messes something up but that makes no sencegiven that everything works fine sometimes.
ok here is what I do know.
The gpu will always work when placed into the top slot alone but not when place in the bottom slot with another gpu in the top slot. unless it wants too that is.
I dont know why but the problem will just go away sometimes normally after I have shut down my computer and then for some reason on reboot the issue will just go away or reapear but this is after countless past reboots with everything working perfect or being broken. I have over after this last time of it working (2nd or 3rd time it has started working after haveing not been) clocked both cards to 969 and I have run that for a few weeks no problems no heat issues but like i said at some point i turn off my computer and the problem comes back. Im out of ideas and I can still RMA my Gpus but im kind of at the point were I have no clue if that is the problem.