What is going on with my computer?

At random times areas of my screen will get slightly pixelated and if a video is being played during this happening the fps will drop. It isn’t always related to video playback or to heavy loads. This recently started happening out of no where a couple days ago and I’m not exactly sure why. The other thing that is pretty odd about it is if I move say another window or something over that area of the screen(causing it to refresh) the problem fixes itself right away.

CPU -Intel i7–2700k
16GB RAM
GPU- Radeon 7870