Screen problem when viewing video

I have installed the latest version of Ubuntu (Jaunty Beta) on my Aspire One, after successfully using Ubuntu Intrepid. On each version of Ubuntu I had an intermittent problem when viewing videos so I thought I would ask here and see if any of you have an idea.

The problem is that after a period of time (this can be minutes or hours) the screen goes black (or sometimes white) when viewing a movie on an external monitor. I have not checked if this is the case if the external monitor is not used. Basically the movie will be playing fine and then without any notice the screen will stop displaying the video. Audio will continue playing but the keyboard will now be unresponsive (pressing the usual keyboard shortcuts do not stop the movie). The only way to shutdown the computer then appears to be to hold down the power button.

I usually use SMPlayer as my media player (as I like to be able to resume movies where I left off rather than having to skip through and find the place again and this is the only player I have found that does this). I believe this is essentially just a frontend for mplayer so hopefully the problem does not arise from this.

I have tried VLC as my media player but this has the same problem that the video will stop being displayed after a period of time.

Any ideas? Thanks!