The problem you are experiencing is generic to most, if not all FX series cards. I also have the same problem on my 5900 ultra. everything is fine until polygons randomly start flashing and the framerate will drop by about 75%. the only way to fix it is to reboot.
Updating the vga bios probably won't help - i've got version 23 (i think version 24 is the latest - whitebunny
the problem appears to lie in the pixel shader 2.0 hardware and its implementation in software environments. There is no known fix for this as it is a hardware fault, and early indications show that it is present in the 6800 series as well.
nVidia seem to be sticking their head in the dirt over this matter and haven't released any information about this whatsoever, as far as i know they haven't even admitted there is any kind of problem whatsoever.
subsequently, games such as Asheron's Call 2, F1 2002, and NFS Underground may be rendered unplayable due to the FX family's inability to process pixel shader 1.2 processes correctly.
probably not what you wanted to hear...