Usualy, when you start an online video, it start playing before it's loaded, at only a few % of transfert, maybe only 1%.
If your bandwidth is not high enough, it will stop as it need to wait for the rest of the video to get downloaded.
The only solutions are:
Use the pause button to stop the video and wait for the progress bar to fill up (fill up the video buffer).
Use a lower quality setting (less data to get).
Get a faster broadband connection (get the data faster).
Electro, May 2012