On a settopbox I have a TVOUT working fine with XvMC. But not all of my friends sharing Debvision (a Debian clone) want to use it with TVOUT but via analog Port VGA on their TV. Unfortunately XvMC is only supported on one port at a time. So I tried to change the xorg.conf to setup single head, setting the portoder to 5 (analog CRT port) and the DisplaConfig to "1". But still the movies that look fine on TVOUT judder/stutter on CRT.
It looks like xserver finds the TVOUT active even though I don't configure it in the xorg.conf and then claims XvMC to this DVO. (2).