Computer issues.

Okay where to start... I have been experiencing problems for a while now and it has left me exasperated. My computer was fine when I first purchased the parts and put it together. A few months later issues started cropping up whilst playing games. The computer would lock up with a black, gray, or even green screen, vertical white lines most of the time, and looping sound which required a hard reboot. I thought the GSOD(Gray Screen of Death which is what I described is called) was due to my XFX Radeon 5870 as I have heard the 5800 series had issues with the GDDR5 ram. The problem started to become very frequent to where it would constantly happen when I was playing a game(upon loading game up to not happening for a few hours) and sometimes it would even GSOD during video playback. It would even fail to boot past the window splash screen sometimes.

I took my tower to NCIX as all the parts were still under warranty. I left it there for a day or two because he said he would test my PC. I later phoned NCIX and a tech guy told me he tested my card with several benchmarks and said my system failed a few of them with my 5870 and used a loaner card in my machine and it worked. He said the 5870 is discontinued so they could not give me another one and said my only options were to send it to XFX for repair or buy a new one. I decided to send it to XFX and they sold me a loaner card (Radeon 5770) for the time being which he specifically said he used in my machine for the testing and worked fine. I brought it home and plugged it in and was immediately confronted with problems. Now whenever I do anything it causes the computer to hang and make the applications stop responding for 30 seconds to 2-3 minutes while I can still move the cursor around but do nothing until it starts responding again. The screen even went black when I tried to load a full screen game but I could close it, but once I could not do anything except move my cursor around the black screen so I had to hard reboot. I am kind of lost as to what I should do. I suspect that it could be the power supply, the ram(I tested the ram using memtest and found no problems), or even the loaner card as well but I am not certain.

If it is the loaner card then I would be even more annoyed as it implies that he did a half-arsed job troubleshooting my PC.

PC specs in case it is needed:
Windows 7 Home Premium 64bit
Intel i7 930 @2.8GHz
G.skill 6GB DDR3 Triple channel memory
WD Caviar Black 1TB 10krpm
Radeon HD 5870 1GB DDR5(currently 5770 due to the loaner)
Corsair TX750W
Gigabyte X58A-UD3R