Either watch videos on your TV using a hardware DVD player, or give us a little more information so we can help.
What model DVD drive are you using, what are your computer specs (it might just be too weak to keep up), what kind of videos (from a DVD, files on your computer, or streamed over the web)? What video player software?
What are you running at the same time? Can you bring up the Task Manager and see if your CPU usage is high?
I'm sure that there is other information that would be useful, but this is all that occurs to me at the moment. Thanks.
Edit: Have you updated your ATI drivers recently? If not, try doing that as a first step.