fedora 10 weird mplayer playback

[Janez Košmrlj]

I upgraded to fedora 10 about a month ago. And since then mplayer 
doesn't play videos smoothly. The sound is ok, but the video plays to 
slow for a couple of seconds, then it plays to fast, to catch up with 
the sound, then it slows down again and so on.
I did a fresh install from the live cd, but i reused the old home dir, 
so the user settings stayed the same.
I installed Mplayer from the rpmfusion repo. I use the x86-64 
distribution, on an AMD Pheom II X3.
My Brother has similar problems on a completely fresh install (including 
home dir) on a core 2 duo laptop.