I've got the exact same issue as you with the XFX 5850.
My specs are
Corsair 6gig ram @ 1600hz
Asus P6X58D-E Mobo
Corsair HX-750 PSU
G.Skill Pheonix pro ssd
Samsung 1tb f3
Win 7 Pro 64bit
It seems a lot of people have been having these issues, particularly with Sapphire/XFX 5800 series. I've attempted many things like different drivers, making sure mobo drivers are up to date, nothing seems to work..
I've also tried installing XP and the same thing happens.
If you find a fix let me know cause otherwise I'm gonna RMA it also