My friend bought a 5870 about a month ago he started recently having problems with it so he ships it back and they sent him another in about 2 weeks. He said that this one is even worse. Could it be that his bios needs updated? Sorry I don't have much detail.
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Did your friend have an Nvidia card before he got his 5870? if so then he most likely needs to use driver sweeper to clean any remnants of the Nvidia drivers.
Do you know the specs of his computer? motherboard, cpu, psu?
Actually yes he did have a nvidia card b4 it I think he at least put the cards in and out about 3 times once to actually try it out second to ship it back and third time to put it in again. So he probably messed it up even more