I've been jumping between nVidia & ATI since my old powermac days... My current rig is 6800GT SLI.
ATI cards have always been touchier for gaming and require more thoughtful usage... probably why all the zombies that don't care properly for their windows installations / driver sets will tell you to run to nVidia. In my opinion both are fine once properly understood.
As for your particular problem: my knee-jerk response is that you have a hardware issue. This could be solved by flashing the card's bios, or by returning the card. Where did you get it if I might ask? There are bad cards floating around out there. Also, are all your drivers & bios's uptodate? Mobo, network, etc.
I would also suggest you try these:
http://www.omegadrivers.net/If you update all your other hardware drivers, load the omega driver, and still get the same error that methinks you have a bad card. Sorry