Because it's not stable... just because it can pass heaven doens't make it stable. BF3 is pretty great actually though for finding those instabilities.
3dfx was a company that closed in 2002 that revolutionized the GPU, in a month after the closure a GPU would have been ready that was the same as the geforce FX and ATI X19xx series. You probably haven't been in tech long enough to get what I said...
Try boosting the voltages a bit.