Thanks for your help
1) The videos are Windows Media Player Files, they were created by Windows Movie Maker.
2) Yes I am playing the videos full screen, but I have tried small screen too and the problem persists.
3) I have tried different WMP videos and the problem still occurs. Eg I have tried the sample video file that is provided with WMP.... same problem. The strange thing is that sometimes the files work and sometimes they don't - doesn't make sense to me!!
4) Yes I can still hear the video playing. It shows the image briefly, then cuts out and I can only hear audio.
Thanks!!!