Random patterns all over the place on screen. Activity monitor shows cpu usage is 80% idle... not sure what's going on
Okay... so I found out what the issue was. If you're using a thunderbolt dock and one display cable is connected via the thunderbolt dock, you cannot then connect the other display directly to the macbook. If you do this, the drivers are not capable enough to prevent the pixel tearing effect due to the pictures being slightly out of sync. Once I connected both displays via the thunderbolt dock, it worked fine.