[mythtv] Problem with XvMC

[Jerry Rubinow]

When I try using XvMC, I consistently get low frame rate and audio
skips.  If I use libmpeg2, no problem.  mythfrontend -v playback gives
repeating cycles of:

prebuffering pause
waiting for prebuffer
video is 3-4 frames ahead of audio, skipping a/v wait
video is 3-4 frames ahead of audio, skipping a/v wait

Every so often the frame rate is reported at 56-58 fps.  Other
settings are deinterlace bob, open gl vsync, realtime priority
threads, extra audio buffering.

Source is 1280x720 over firewire.  XvMC driver is 7676.  Myth version
8906M.  FX 5200 card.  CPU is low, around 50%.

I'd like to try to understand possible causes of this and either give
any helpful information to someone who might be able to fix it, or
accept any helpful suggestions to try to further diagnose it myself.

Thanks,
-Jerry