Dude! That would be heat! I had *exactly* the same problem with my old VGA (8600GT) it's core and mem were pushed to the edge, So I had to bring down the clocks a bit (Specially with Crysis, other games were fine) :)
It's an FSP 700W as he mentioned, so it is out of the equation, and those ppl have checked the VGA with an ASUS 750W something :D
btw he may be on XP not Vista.. hehe