I generated a full HD 1920x1080 mp4 video file and tried playing it back using Quicktime. The playback was very jerky. The same file played beautifully using VLC. So I don't think it is cpu horsepower issue. Anybody have similar problem playing back mp4 HD materials using Quicktime? Any ideas? Mac Pro 3.2 Octo with 10 GB Mem and 4.5TB diskspace.