That unit is about as chintzy as they come. No reviews, at all, nor any references to it, which is always a bad thing. When the model number only brings up a very few obscure listings at places like hookbag.com, buzillion.com and one Amazon listing, with reviews like these, it's very bad. I've never even heard of the brand in over 25 years of working with computer hardware.
http://www.amazon.ca/Supply-EPS12V-SLI-ready-PCI-Express-KENTEK/dp/B00596U0YQ
http://www.tomshardware.com/answers/id-1963050/kentek-good-quality.html
That PSU is made by Kentek. If you google THAT, all you will find is posts about failures and explosions. And hardware damage. I'd say you've been lucky so far, mainly because you didn't have any hardware that was placing any realistic demands on your system. Once you installed a different piece of hardware, it likely pushed that POS over the edge. That being said, there could always be other issues going on as well, but since it's a fact that you've got a problem waiting to happen, or that already happened, with that PSU, I'd probably address that first.
Clearly, you got lucky for a while, and now, not so much. Replace the PSU first and then determine if anything else was damaged by it. At the very least, try another PSU to see if the problem resolves itself. Pretty hard to believe that two different GPUs have the same issue. There may have even been damage to the board.