Which version of Quicktime are you running??? 3, 4, or 5?? Standard or Pro? I know that the Pro version unlocks features. You might want to consider purchasing the upgrade from Apple (something like $30 or so). Also, when you open up Quicktime, under the Help menu (under OS 9) or under the Quicktime Player menu (OS X), there is an update software feature. Run that and allow it to update what it likes. I would also suggest running the software update control panel (if under 9.x) or the same item from the system preferences (under OS X).
You can also try importing the mpg files into Quicktime, and see if that helps any, or just opening them from withing Quicktime.