I have a mac pro with Leopard 10.5.2 and Quicktime 7.4.5. I have both Perian and the Flip4Mac plugin installed if it makes any difference. I can't figure out why but it seems to skip and lag when I try to play .mpg files on it. The codec once I look into the file info of the skipping movies is MPEG-1 Muxed. They play just fine in VLC though. Anyone have any idea on why it's happening or any suggestions on how to fix it?