Ron, apparently you never went back to the Apple Discussion forums to look for your answer. A nice lady explained what was happening and how to fix it. A program called Rosetta fixes clips that were improperly assembled, and there are also free programs available to repair mpgs that were created on Windows that will not play on the Mac OS.
Anybody else having this problem would do well to use the Apple Discussion forum for QuickTime for Mac OS. For years I wondered what was going on, but then again, I didn't ask.
But if you do ask, please follow through on your postings and give results. It not only helps you, which I realize is your first intent, but it can also help others who struggle through the same issues.