Try this. Remove the heat sink and fan from the GPU, remove the old thermal paste and put on new pastse (or replace the fan: Newegg). I had a 6600gt that was runnig at 100 degrees and simply replace the thermal paste and the temp now runs routinely at 52 degrees. If it is a BFG card you can simply RMA the card and they will replace it no questons asked!! Try the thermal paste refresh and see if the artifacts go away. sounds to me like tghe card is overheating.
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.