If it is only certain videos and only through youtube, then I would start by checking the same videos in other browsers and see if happens in them. Also, you should go to any of the videos, when it is doing it, and right click on the video. In the popup you should get an option for "Troubleshoot Playback Issues". Type in the info on what is going on and include a screen shot. This way youtube can look into it as well.
You should also clear out your cache and history (make sure you save anything you need and/or want first) and then restart the browser. See if this clears up the issue.