Yeah, hardware issue if problem is in BIOS screen ..
Two things to test first :
HD4850's fan/heatsink assembly. If it's never been cleaned, you can take it out and make sure it's not an overheating issue due to dust build-up in the fan/heatsink fins .. If it's out of warranty and you're able to do this yourself, then you could also check the thermal paste situation and maybe replace the TIM ( as the stuff does sometimes cake solid or dissipates over time ).
PSU check too : make sure voltages @ 12v are within spec.
If card has been cleaned and PSU is still OK etc, then you could always bribe a buddy with a few beers and see if somebody will let you test it in a secondary rig .. ( assuming their system has an adequate PSU to power the card ) ..
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