Originally Posted by sortie

The player is most likely the problem.

I've built a few flv players and they all can do things differently.

I would guess that your user uploaded video is being converted to flv at a
higher quality and your server may stream too slow and the player is not
handling it.

Put the user uploaded vid on a fast server and see if it works better.
If it does then redo the user uploaded video at a low quality on your server.

Try a different player.