[matm347]

I used ATITool to help me when I was overclocking the video card in my XPS2. It will scan for artifacts and bring the GPU up to max operating temps.
I use stress prime to check for CPU and RAM problems.

As far as different games causing the problems, each game will stress your system differently and even stress different parts of the subsystems differently(ie, heavier shader pipeline use in some games).

I suggest running ATI Tool and StressPrime at the same time while watching temps and looking for artifacts. If you can do this overnight then your hardware is fine.