Whenever I try to watch video files on my Mac (no matter which computer I'm using), they just don't play smoothly. They're all choppy and everything . I see this happen with videos that are encoded with the H.264 and are in an .mp4 format. But, if I import the same file into iMovie, it plays flawlessly. Are there any tips to help improve video playback quality when I use Quicktime or iTunes?