Mplayer runs slowly, screen split diagonally

[John Talbut <john@dpets.demon.co.uk>]

When I try to play a DVD using mplayer it runs slowly, something like 4 frames a 
second.  The screen is split diagonally and it seems as if one frame is pasted 
into the top left half of the screen and the next into the bottom right.

Any ideas about what is happening, how to diagnose it or how to get it working 
correctly.

Currently I am using the latest Debian testing distribution packages, though I 
have had similar problems with etch.

John