PC Freezing when playing games..Out of options...

I am not sure this is the right forum to be posting in for this issue... I am going for it anyways =)

I built myself a pc out of a mix of new and old parts at the weekend I shall state which are old and new in my little list below.

My Specs
ECS p35t-a Mobo (Old)
Intel core 2 Quad Q9300 2.5Ghz (old)
Seagate 500Gb 7200rpm HDD (new)
2x 2gb Ram (not sure on the specs of this. (old)
Thermaltake Tough Power XT 675w psu (new)
XFX Radeon HD 5830 1Gb (old)


I think that is all. basically I am stuck at a dead end with this I have tried everything that I can to try and get this to work successfully during games. a game can be on for no longer that 10 minutes before it freezelocks my pc no mouse movement or keyboard response. I have checked the wiring so many times replaced the gpu with a different one, replaced the ram to test that stressed the old ram and new, stressed my cpu, stressed my gpu, changed my heatsink.

I am so stuck and soo annoyed with this whole project I have basically come to the conclusion that it's a motherboard fault but I have had no problems with it before hand. I would like a second opinion on this as I have no spare motherboard to test in this build.

All help would be much appreciated and I look forward to hearing some responses =). Thanks in advance.