Instability problems with Pinky

[Sandmaster500]

Some will know the mob i did to my mums old PC named Pinky (guess why).
Well inside there is:
1.7 celeron
640mb RAM
9800 PRO
Gigbyte GA-8ID533
80gb WD
Tsunami 400W PSU

Well 'she' if fine at home runnning for aslong as you please. But at LAN parties about 30 mins or so into a game the screen will go dead and the only solution is to restart

My guess is that the GPU is overheating. Is there anyway i could test thing/any other ideas for the problem?

Thanks
Sandmaster500

 

[Sandmaster500]

Anyone have any ideas on this one?

I'm thinking its either the PSU can't cope or that the GPU is too hot/on its way out....