Computer hard freezes during games - 8 month problem

Hello,
I am in serious need of help!

I built my computer 8 months ago and have had this problem since day one.

The problem is my computer will hard freeze when I play games, taking anywhere from 5 minutes - 2 hours to do so.
On the more graphically demanding games usually freezes early on, Skyrim for instance freezes within 5- 10 minutes.

The first thing I did when I noticed this problem was purchase new RAM, I bought 2 x 4GB corsair vengance low profile 1600mhz.
Obviously that didn't solve my problem. So I tried running a series of tests that people suggested, e.g. Memtest, prime95, GPU stress tests and many others, its been so long I cant remember them all but none the less no problems showed up.

So I took my pc down to the pc specialists in town, who told me its most likely my GPU however could be my PSU.
I bought a new PSU after being told my PSU was a no name brand anyways and I bought the Corsair HX 750W, but that didn't help.

Then the problems started getting worse. I was getting blue screens, failing to boot and this random thing when I leave my pc for a while it goes to sleep mode? or something like that so if i move the mouse it would come back on but instead it attempts to come back on but turns itself off then on again for 3 secs then off then on again etc untill it finally tries turning on for good but there is just a black screen. I also noticed upon failing to boot the VGA LED is red on my mobo. So obviously I was led to believe its my GPU. So I got a friend over, uninstalled my EVGA GTX 560 ti 448 core and installed his ASUS GTX 550 ti. I was able to play multiple games simultaneously including Skyrim on max graphics for around about an hour, when I decided this is more than proof enough that my GPU is faulty.

So I sent away my GPU for RMA, but did not unistall the drivers as I thought I would get the same card back, however I was sent an EVGA GTX 570 HD instead.
I installed the card but cant figure out how to remove my old card drivers if that's even necessary.
Also when I tried updating Nvidia drivers to 310.61 the graphics drivers installed but the update part was failed to install. I tried installing the old one I had 306.97 but some things failed during that too.

Anyways the same problems still persist. I Tried a stress test on my GPU this morning and after only 4 min 30 my computer hard froze. GPU temp 80 C , GPU load 97% no artifacts or anything like that ever.

My specs are as follows

Mobo: Asus sabertooth p67
CPU: i7 2600k 3.4ghz
GPU: EVGA GTX 570 HD
RAM: 2 x 4GB Corsair vengeance low profile
PSU: Corsair HX 750W
Windows 7 ultimate 64bit

If you have taken the time to read this and comment, Thank you so very much!