Just recently purchased a 2010 5,1 upgraded with a Quad core 3.46 X5677 processor. I'm currently running El Capitan. Wanted to upgrade the GPU from the stock HD 5770 so did some shopping. First card I bought was an AMD Radeon R9 270 2GB, but could not get anything on the screen (Apple 27" LED w/mini display connector) despite trying everything I could think of. This was surprising since I've bought from the vendor several times before and never had a bad card or any other defective hardware - but there's always a first time. So, bought a Radeon HD 7970 3GB card from a different vendor. At first, I got the same result at first but after a restart or two with the power button I got it working. Keep in mind I'm no Mac hardware expert. At all. By any stretch. Every now and again, however, about maybe maybe 4 or 5 time now since I installed the card about 2 months ago, the screen will either go black, or I'll get screen static/stutter. It only happens momentarily and then everything is back to normal. The stutter only lasts for a minute at the most, and a restart gets rid of the black screen when that happens. The two different issues never happen at the same time. This only seems to occur when I'm pushing the card a bit with graphics intensive games like Rise of the Tomb Raider. It's happened both in Mac OS and Windows 7 in Bootcamp, though I've not yet got a black screen in Bootcamp. Is it the card, or considering the issue with the first card, do I have a dying PCI board? Can I try the card in a different PCI slot? Is it something else? Thanks in advance for your help.