if updating the driver doesn't fix your issue, then it could be a bad video card.
let try some ideas:
have you updated the agp drive to the latest version (see system board maker for driver update)
In the bios, :
chang AGP speed from "auto" to "8x"
turned off AGP "fast writes"
now, a silly question. have you taken "canned air" and blown out the heatsink and fan on the card? could be that the card is overheating when placed under game load.