I have a 9800GX2 and have ran Vista 64 bit without issues such as you've described. Have you tried a fresh install of vista 64 since the issue started happening? What about making sure both floppy drive connectors are connected to your mainboard for extra power? Are both of your power leads (2) connected to the top of the card as well? Have you also tried to swap slots the card sits in? Did you check to make sure the green LED on the back side of the card is lit confirming the 2 power connectors are right?
I did have an odd FPS issue that would come up out of nowhere, usually 1-2 days, and every game i played was -30fps from what it was previously and the screen would turn to utter chop. System restore back to original install and same. I'd create a restore right after i verified my frames were silky smooth too. I went to XP 64 bit and haven't looked back and everything runs better =) I'm pulling 91FPS in COD4 @ 2560x1600 on my Samsung 30". S i L k Y!
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