@Socket54 , i think you are having problem with the power draw...check cables and everything....if you elemenate all problems with cabling, try mesuaring inside you PSU, capacitors etc with multimeter

@CharNoir , try with significant downclock on the memory of the problematic card, or maybe slight downclock, you can check what are the best settings after a while...for exemple, my 7970 is stable on 1300 Mhz (patched bios) , but sometimes i can run for hours with 1315 Mhz....