Blackbird:

Have you seen the issue for yourself? It's weird, and it bugged me for a long time before I stumbled upon a wise woman on the Apple boards who guided me through.

You get a large, maybe 100 MB mpg file, hit play, and the play bar moves much quicker across the field than it should. Then, if you manually move the playbar to the left, you see parts of the video that did not play during normal playback.

Now, whether Rosetta would work on an incomplete file adds more wrinkles in theory at least. I think Michaellloyd will need to see it work on a complete file that is exhibiting the strange behavior and then try and deduce what is occuring.
Can you apply the Rosetta 'fix' to a file in the process of downloading? Don't know, but I tend to think not. I haven't had it happen to me, so I can't weigh in other than this guessing.
Maybe preview just enough to see if it is a file you are interested in, then wait until you have the whole thing to either play it or try to fix it.