mplayer stops after <1sec when streaming

Hello,

I've been trying to view the video news on bbc.co.uk, and other embedded video-streams elsewhere on the net. I have mplayer and mozilla-mplayer installed. When I try and view one of these streams, mplayer buffers it, and plays around half a second of the audio, sometimes accompanied by a few frames of the video, then returns to "Stopped". This doesn't seem to be limited to one type of stream; I get the same behaviour with ogg theora, windows media, real and quicktime streams.

Why does it do this, and how can I fix it?

If I download the file (only possible with some), it plays fine in the mplayer application.

This is Ubuntu 6.10