leadtek pcie 6600gt artifact problem

[domineex]

I was playing the game chronicles of riddick for about one hour or so, some artifacts occured and the computer crashed. The card is on it's default values (500/1000). After this I played the rthdribl demo for a while and no artifacts occured. N-Bench 3D test gives no errors on the image. But in games (i tried chronicles of riddick and nfs2 underground) and 3dmark05 again artifacts occur. While I'm running rthdribl and n-bench, the temperature was 57 degrees at most. What may be the problem?