Try setting the AGP to 2x in the BIOS and disable fast write and the 2 other options in the AGP part of the BIOS and see if that changes anything. Do them one at a time and perhaps you will be able to ferret out the culprit. My thought is perhaps it's in the BIOS. What version are you using, anyway? Try
2929 BIOS as this is an agressive one for memory timings.
Just a shot in the dark! I know what it's like to wring your hands and mutter at the PC!
