Screen bug - Yellow pattern all over application windows

I've been using my iMac (mid-2011, OS X Yosemite 10.10.1) without any problems since I purhaced it. However, today a distracting yellow square pattern started to appear. It only appears over application windows, ie. I can't see any pattern over my desktop, but my browser is covered with it. It doesn't appear on every window, but almost all of them. The pattern moves if I scroll and it does change if I refresh a webpage. It also appears on screenshots I take.

I've tried rebooting and updating my OS without any change.

Example screenshot:

What could cause this and is there anything I can do to fix it?