Gpu gives up when I start game

I have a GTX570 which has run finew for over a year. All of sudden it starts giving me problems, failing to post, crashing in games etc. Local tech tells me its a GPU issue. Replace GPU for a Radeon 7870. Works fine for a couple of days then as soon as I remove the GPU to test the returned 570 which Inno3D tells me is fine it starts having issues. The returned fine 570 also fails to post. The 7870 will posty and start PC running Windows, browsing etc normal but if I try to return to Skrim I get a blac screenwith sounds before it boots me out. Similarly #D mark dies in a bout 1 second.

I believe the cards are fine so could it be the power? I have 2PCIe 6 pins. I wonder if one of them is dodgy?

Any thoughts?