Fuzzy pixelated blocks appearing on screen

Hi all,

I haven't managed to find this happening to anyone else. It only happens in Illustrator.

These blocks break up the image on my screen as I'm working, and I have to zoom in and out to get rid of them. Does anyone know why this is happening?

I'm running OSX El Capitan on a late 2013 Mac Pro through 2 monitors. (A Dell, and ASUS 4k monitor - where i have the issue)

Any help would be greatly appreciated.