Our videos are encoded in a high quality mp4 format.  In rare occasions, some players will not deal with these HD files efficiently.  If you're having problems with your player, try the free VLC player.  It works with just about every format in existence and is more reliable than most commercial players in our humble opinion.  It can also slow down our videos to help with transcribing, which is an added bonus :). Get it at http://www.videolan.org/vlc/index.html