I have also seen this before on some mac mini systems. The content will not be dropping frames at all and the meters are barely pegging 20% and there will be some serious horizontal lines and such in the video. It is much worse if running remote desktop.... It seems to be something that the video card can't just handle it. It's not a content, drive speed issue for sure as I can play back 8 layers without a skip, but it is something I have noticed... All videos in pro-res proxy, definitely the ones with higher data rates and faster content though.
