Do you have a 60 Hz display? If so you're probably hitting one of the limitations in svn MPlayer's VDPAU support; it can't play video with FPS higher than display refresh rate (and in practice near-equal values already cause problems).
Compiling a player from
should avoid that problem. There are also working Ubuntu packages available at least at
(the "coreavc" name is there because the PPA was originally created for CoreAVC support in MPlayer, but you don't have to use that functionality).