Intel Mac :: Quicktime Playback Distortion On Some Mp4 Files?

I am having a problem playing some mp4 files through Quicktime. When the video is played in Quicktime (or Pro Presenter 4), portions of the video turn into distorted white blocks that move with the video. It doesn't continue throughout the whole video, but I can't discern when or why it only affects certain portions. It does it every time at those places in the video. I downloaded VLC, and the same exact file plays perfectly fine in VLC and on other computers through various programs. I have only seen this problem on this computer. 

Is this a codec issue? How do I fix it? The files in question are mp4 files using h.264 video and AAC audio at 1920x1080 resolution. I can play other mp4s using h.264 and AAC without any problems. 

I have only had this problem with one of the companies that we periodically hire to produce videos for us. I'm guessing that it has something to do with the way he encoded the video, but I have no idea what the problem is. It's probably not his fault if it only distorts on this one computer. 

Here is what the white block distortion looks like: 

Info:
iMac, Mac OS X (10.6.8), 3.2 GHz Intel Core i3, 4 GB RAM