Just to try since you seem to be outa ideas..
Try yanking everything else outa the machine.... just run the cpu/motherboard/ram/videocard/hardrive/optical drive.....
Install vista x64 fresh and then install your chipset drivers, then update windows to the fullest, after which download the latest 64bit ati drivers and install them..... see what happens after that...
Hopefully it's not a glitch in the way the motherboard is running the card..
I've only seen 2 instances myself where a specific type of video card with a specific motherboard has causes weird issues either similare to that or worse (no boot at all even).... 1 case however was resolved by the motherboard manufacturer with a bios update..... It happens
But give that whirl, that way your eliminating anything else that could be possibly interfearing with the card..
ALSO try and make sure you use FAILSAFE settings in the bios or try Optimal settings in the bios....