Michael,

Some mpgs that were created on Windows exhibit a weird pattern on a Mac. If you launch a file, and manually move the scroll bar to the end and then back a bit, you may see different parts of a video, yet it won't play properly if you just hit the play button. Segments are obviously not being played.

There is a shareware program called Rosetta that will 'fix' and 're split' these problem videos. You end up with more files, but at least you can watch them.

This 'may' be what is happening to you.