There must've been a problem when you downloaded the video, or there is an issue with your player.
Personally, I've downloaded the mp4 to different devices and all play perfectly.
Thus, do the obvious and re download to your device(s).
If you're having the problem watching the video here at the PM site, make sure that your flash player has the latest updates.
Finally, check your browser...try a different browser to see if the problem still exists.
Good luck.