That a freshly installed Win doesn't help indicates again that hardware is involved.
What's the make/model of your mainboard? I've read already about boards that don't have an option to turn off fastwrites. Welcome to the extremely user-unfriendly world of graphics cards vs. chipsets..:sad:
But sometimes these settings are well hidden only.
You can try "Coolbits 2" to unlock the hidden pages in your Forceware driver control panel (download somewhere on Guru3D.com/ google for "Coolbits"). AFAIR there are "AGP settings" and "Troubleshooting" menus after you ran Coolbits, where you can tweak some things. Unfortunately the old control panel is gone with Forceware 91.47 and so are these menu pages. I didn't mention this before because disabling FW there is a) not the best place and b) doesn't work for some boards (checkbox becomes ticked again after restart).
Another thing to check on sporadic freezes are the chipset drivers. Get sure they are installed and the latest available.
Depending on age of and chipset on your motherboard, there also is a chance that this particular mainboard model doesn't run stable with AGP8x. This could be solved by the flashing the latest mainboard BIOS available, but sometimes there isn't any newer BIOS. If all fails, try reducing the AGP rate to 4x. This helped some people, too.
Keep in touch... :)