I repeat Mplayer's Man page is one of the all-time great man pages.
Spend some time with it.

Thanks all. Solved problem; for the record:

Using Debian Lenny 5 backport up-to-date. 2.6.30-bpo.1-amd64 .

MPlayer would not play *.mp4 files satisfactorily. They paused every third of a second or so, then ran on, paused then ran on; both sound and vision. Sort of stuttered. This also happened with the browser mplayer plugin.

Slightly elaborate fix:

Uninstall mplayer, apt-get remove mplayer

Then get hold of the latest mplayer: svn checkout svn://svn.mplayerhq.hu/mplayer/trunk mplayer

Then install # apt-get install libxv-dev.
then:
# apt-get install libvdpau-dev and also libasound2-dev should be installed for alsa at some point.

Then compile mplayer, this is for Debian, in the mplayer directory created by svn; subversion needs to have been installed:
# DEB_BUILD_OPTIONS="--enable-runtime-cpudetection" debian/rules binary

No idea if --enable..... is a good idea - the thing is just a method of giving options to configure, e.g. --prefix=/home/fergie

Then install the *.deb file that has been created in the home directory usually.

EDIT: Am told installing xv is bollocks because already using nvidia proprietary binaries. All can say is above is what seemed to work.