Hey there.
If the complete file plays, then we are probably back to the original responses. It's not a Rosetta situation or solution. If you ever come across a complete file that acts up, now you'll have a tactic to try.

Another player that does an excellent job handling lots of different codecs is called
MPlayer. Free!!
I also installed Divx 5.0.x, and the mpeg2 codec that Quicktime offers. I have pretty good luck now with QT 6.3, but I still use MPlayer for problem children. QT is not completely there yet, but it is important to Apple, so it will continue to get better.

Good luck with your downloads.