@Alejandro:It will help the C9 if you can let them know which of the three players you are using that you are having the problem in. There's the HTML5 video player, the smooth streaming Silverlight player and then the downloaded video files (in their many formats). All are encoded differently (yes, there are that many different physical versions of the one video) and there's always a chance that one of the encoding went wrong...
Use the Format button under the video to select your video format or the download buttons next to the video...