[sarcasim] don't use a 486 [/sarcasim]
seriouly though bro, try to intstall all the drivers again, checking 3Dmark after each one, then you'll know what happened, in case it happens again. If that doesn't work, ya might wanna take a look at the display settings and see if something is set out of whack, might be a glitch in software somewhere. Checking the heat might reveal the problem too.
If none of that brings it back to normal, try checking some other system settings. Check CPU/RAM/Mobo benchmarks and temps.
Other than that, an RMA might be in order. I just RMA'ed that was showing garbage while playing directx 8 games. I never seen how it affected 3Dmark, but my retailer took it back and gave me a new one no problem.