Probs with radeon X800 GTO 128mg AGP

Hi Chaps,
Firstly this is my first post @ Major Geeks so Hi to all and please treat me gently! O.k ..Recently whapped in a Gecube radeon X800 GTO 128 mg AGP card into my p.c to replace the radeon 9600se that came with the system -

AMD Athlon xp 3000
1024 ddr ram
xp sp2
(need to know anything else?)

Got it in fine and the result were great ,fps up etc but within 20 mins of playing hl2-dod-cs etc I get a blue screen (no error message) and have to reboot. So far I have tried

updating the latest ati drivers & catalyst control centre
uninstalling previous and current drivers and re-installing again
increasing the vga ram voltage to 1.6 in the bios

Still blue screening!, I thought it could be the psu not supplying enough juice (300w) but this apparently meets minimum requirement in the spec - could this just be a faulty card?