I have noticed several movies/videos tend to be blurry one moment, then clear the next. For example you're looking at a man's face. While he's speaking and the film is playing, it loses focus,then comes back in, then goes back out. This is not a very fast or pulsating rhythmic effect, rather it's a bit random. I see this often with .wmv code type *avi or *mpg/mp type files. Any idea what causes this?