Video stutter with iTunes movies - how to fix?

I just downloaded some movies from iTunes for the first time in both HD and standard format but none of them will play stutter free. I have these horizontal lines going through the image like the frame is slowly building itself up. Sometimes the whole frame will even freeze and then fast forward to catch up with the audio.

I never had this problem with any other HD video on my computer before. Does anyone know how to fix this?

Thanks!