Did you do like I suggested and step through the video frame by frame? Yes, did not see anything.
Is it possible for you to post this video somewhere? I will check w/ our IT guy.
Does this happen when you play it back on other computers? On a Windows box, using the settings you suggested (1.2Mbs) it looks ok, but not on the Mac
Does it drop/skip a frame in the same place each time and with each different encode? No, different place, different with each encode.
Have you double checked that it isn’t a problem with the source file? The source playsback fine.