ATI HD4870 freezing

Hi there,

I've recently upgraded from an NVIDIA 8600 to a ATI HD4870 card, and am now experiencing frequent system lockups.

Currently it will last 30 mins at the most gaming (Warhammer online) before locking up.

My system is:

AMD 4200+
3 gig memory
600 watt psu
Asus A8N SLI Dlx board

I've monitored the card temperature and that is fine, the drivers and whole OS have been re-installed but the probelm persists. I can pop the NVIDIA card back in and the system performs flawlessly, but its plagued with hangups with the ATI board.

Ideas / known issues with this combination?

Thanks