I have my Radeon9600Pro AIW overclocked to 490/360. I occasionally get a thin black line appear on the screen for like half a second. It doesn't do this when it's clocked at 400/300. It runs, however, with no artifacts at all clocked at 520/380 on everything else such as 3Dmark, Halo, CS, Homeworld2, or even looping vertex shader 100 times on 3Dmark. What could be wrong with it?