Could be caused by corrupted DirectX files. Download and install this. It will overwrite any bad files.http://www.microsoft.com/en-sg/download/details.aspx?id=8109
If that doesn't work your GPU could be unstable. Even if you didn't touch the clocks it is probably factory overclocked.
Get it to run at 1000Mhz core clock and 1125Mhz memory clock.