Sounds like PSU issue not graphics card.
Being as you tried 3 cards, and got the same result with each one. I think we can safely say it ISN'T the GPU at fault.
You've also tried two different PCI lanes, pretty safe to say we can rule that out too.
Pixelation and screen tearing followed by a reboot is classic PSU failure symptoms. Is your PSU getting enough ventilation? CoolerMaster aren't known to make the greatest quality PSU's.
Something from Corsair, XFX, Antec or SeaSonic would be recommended.