iTunes shows check pattern during video playback.

Whenever I try to play any 1080p TV show I've bought and downloaded on iTunes, every minute or so I get a flashed image that looks like this:

The image only lasts for one frame and then its back to the video, but its extremely annoying!

I actually timed the gaps between flashes, it went like this:

20s - 1:20 - 20s - 2:00 - 1:00 - 40s - 1:40 - 20s - 20s - 1:40

The flashes appear at different times during the videos every time I play them, so it's not an issue with the video. I've tried playing them in Quicktime and I get exactly the same flickering.

Also, the check pattern will occasionally freeze onscreen with no play/pause controls and the only way to get rid of it is to exit fullscreen by pressing esc...

This is the first time I've seen this issue and, oddly, it doesn't effect any of my other iTunes videos. It seems to be just 1080p iTunes purchases that are affected. I'm on a new late 2012 slim iMac 27" with the latest iTunes. I've tried restarts and reinstalls, nothing seems to fix it. Could it be a graphical issue? Or do I just need to download these shows again? Help!