Firstly, try another form of player. Div X, Winamp, Power Dvd etc, etc. Secondly, press ctrl+alt+del when watching your video and see if the cpu usage is maxxed out. Thirdly, what version of Media Player have you got ? Help - About will tell you this. Forthly, is it a film, music video, or smut that you are watching ? Either could possibly be corrupt in some form, possibly re-download it from a different source

HTH