Artifacts in movies

I was watching a movie on my pc and notice little defects which I though was from the movie but after bringing my clocks to default it was back to normal, but the strange thing is that I don't get any artifact when doing anything in 3D I even got the atitool checking for artifacts which it didn't find.

What is wrong here?

The part of the chip that process video files could be overheating. Remember not all of the core is in use 100% all the time.

For me it was a RAM issue. Video ram was damaged from overclocking..