problem with video - xp

I have managed to get an xp messenger conversation with video + sound
working (Yippee) - but after a couple of seconds it fails complaining of
network error?? (BooHoo)

I have looked through the 'everything you wanted to know list' but cant see
anything

Surely if it were a port issue then the video + sound wouldnt work
initially?

Im on a network behind a dlink 804 router

Any thoughts

Chris