QuickTime player is not the best player for previewing. I'd recommend VLC instead; VLC - (click on link) _ (choose nearest mirror site)
MPeg's can be a little tricky on playback until they've completed downlding. But VLC will definitely do a better job that QT player.

Normally red files in the library mean they're not being shared. But if you see them in the incomplete folder it can mean that the file has been forgotten about & a replacement incomplete file created. Arrange your incompletes by name or type & check to see if there's a 2nd one by the same or similar name.