Playback speed fluctuates

I have recently started using Windows Movie Maker for my video editing on my
laptop running Vista Ultimate.

Once I have published my video and play it back the speed varies and at time
the mp3 soundtrack plays very fast and suddenly slow. This is playing the
video back using Windows Media Player.

I have also burned a movie to a DVD and experience the same problem on a DVD
player but to a lesser extent.

Anyone have any ideas what is causing the problem and how to resolve it
please.

Thanks
--
Hogard Gevers
Pietermaritzburg, South Africa