Gaming machine black screens

Hey,

So I've upgraded my PC a fair bit over the years and have a certain problem... When I play Warhammer Online: AoR, Day of Defeat: Source or Garrys Mod 10 Zombie infection it goes to a black screen after 5-45mins. I can hear sound but it lasts for 5seconds then judders. I then have to turn off the computer rather then rebooting it. I've taken back my GCard and the fella says it's not that (He ran 3D MARK) which is odd really... I've upgraded my PSU from 450 to 700 and still get the problem =/

Here's the spec
AMD 5000+ BE (Not O/C'd) but running at 2.8
4GB RAM
Gigabyte 8800GT 512mb
M2N32-SLI Deluxe
Zalman 9500
EZ-cool 700w

Any ideas?
On the same issue I guess, when I try to run CoD4 most of the time I'll get 9-25FPS then when working fine it goes back to 80-100.... I don't change anything either, it just decided to work every now and then.