I figured out a fix!

Turns out, it was Quicktime fooling itself into thinking the shortened, "previewed" video file was the complete file, and thats why they didn't play.

I loaded up one of the "broken" files in VLC, and they work flawlessly, playing all the way through.

I just figured I'd come back and post my triumph, in case someone else has a similar problem.

But, I thank both of you for your help!