I'm having trouble with a nvidia 5600 video card, after my pc has been on 2 hours the screen go blank and the pc lockups.

The HSF has been replaced with a Zalman fan (looks like a VF700) when I got the card I had it power up for about 45 mins - with no issues but I noticed the Zalman was not fitted correctly, I re-sitted it so it flush with the chip.

I thought the ambient temperature of 37C in the case might be an issue or my low quality PSU -after an upgrade new case, the ambient temperature is 27C, and I have a Seasonic 430w.
I running Windows 98 SE with Forceware 81.85 drivers (the only other thing in the PC is a D-Link DFE-530TX PCI network card).

I'm not doing anything taxing, not running 3d games, just surfing the net when it lock up. Normally this forum via Firefox 1.5.

I know the card worked fine before the Zalman was fitted, but I do not know if the card was powered up for more than the 45min I did while the Zalman was not fitted correctly, could it be 'fried' but showing this symptom? Any ideas?

oh with my Ti4200 the PC runs fine hours.