Computer crashes @ high load, kind of ambiguous problem

So, i've had this build since late 2009. It has:
Mobo - ASRock K10n78 (AM2+, DDR2)
Proc - AMD Phenom II X4 940
Graphics - ATi Radeon HD4850 (XFX)
HDD - Samsung HD501LJ (500GB)
PSU - Thermaltake Toughpower 575W
Ram - Don't remember exactly, Kingston 2x2GB though.

So, since i've had it, i originally had a Palit GTX 260. This worked fine for a day full load and then, on the next day it would crash after about 5 minutes of playing a powerful game (at the time i was playing Borderlands @ max detail 1680x1050). This would be specifically a graphics crash, my screen would change to random lines of colour (kind of like a texture spammed across the screen), i would hear sound continue for a few seconds (for example, i was talking to my friends for a few seconds on skype after my graphics are gone( "What's happened?" "Graphics has crashed.." ) followed by a static noise. I returned this card and bought a entry level card (a HD5450) as a replacement for the time being. This card worked fine and never crashed. I bought a new graphics card after this thinking that the problem was the graphics card (even though i replaced the original one), and then to find that my new graphics card (The HD4850) still did the same, although not as bad as the other one. I then decided that my power supply was the problem, a bad unbranded power supply (although 800W.)
Recently, i bought a new case and power supply for my computer, to find that the same problem still happens. My temperatures are completely fine, with my processor no higher than 60C full load and my graphics card no higher than 68C.

Does anyone have any idea of some tweaks i can do to fix this problem?
Does anybody know the root of the problem? (i'm guessing it's the pci-e slot on the mobo.)

Thanks.