GPU is beginnig to fail. I would uninstall and then reinstall your grahics dirvers first before replacing the card....but I think it's time for an upgrade. Go to an Nvidia 6800GT or, if you are money conscious, a 6600GT will make you happy!!!
Noli Nothis Permittere Te Terere!!
EVGA 680i, Dual EVGA 8800GT'S (650/950), 4 gigs OCZ DDR2 1066, Intel Q9450 at 3.8 Ghz, Koolance Water Cooled, Dual Plextor16x DL DVD+/-RW. Dual WD 250gb 16mb Cache Sata 3.0. 3DMark06 score = 19,168.