This usually happens because network congestion prevents the BBC's servers delivering data to your computer quickly enough.
Your network may be being used by another application running on your computer.
If you share your network with other computers in your home or office, another computer may be using your network.
If another application or computer is doing something bandwith-intensive - such as downloading large files or using Voice-over-IP applications - while you are watching video, your video stream is more likely to play less smoothly.
If you do not believe your connection is being used by other applications or other computers, check your media player has the correct bandwidth settings. See the following help pages for more advice.