It looks like the streams are throttled and the player cannot cope with the pauses between frames. Instead of playing directly from web you can download first then play.
> Highlight the video you want in the list.
> Press the MENU button and then select download
> OK the downloading message
> Press the MENU button again and select 'View Active Downloads'
> Wait for download to complete, or EXIT and do something else. You can come back to the list and reselect 'View Active Download' later to check on progress.
> Play from the 'My Videos' menu item.
