Problem solved. I commented out the XF86Config-4 line "Option NvAGP 1" and the problem is gone. I was using the NvAGP driver with 6629 to solve a stability problem I was having.

I finally tried the nvidia 1.0-8756 driver last night. I used the provided nvidia installer (since the KnoppMyth nvidia script "install-nvidia-debian.sh" was bombing out on me during the update process).

The video playback in Mythtv is smooth - as with driver 8178.
I think this new 8756 driver solved a XvMC mpeg2 decoder problem I was having. In 8178, I could not watch live tv with the XvMC mpeg2 filter on. I got a black screen rather than video. So, I had to set the mpeg2 decoder to Standard in order to see live tv channels.

With the 8756 drivers, I can have the mpeg2 filter set to Standard or XvMC and live tv is fine.

I just looked at CPU usage with the XvMC turned on while watching live HDTV and while watching an HDTV recording. CPU usage is down to 20% during playback and down to 35% with live tv! Wow.

Eric

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KnoppMyth R5.5, Asus A8N-VM CSM (nvidia 6150 onboard video), AMD Athlon 64 dual-core 4200+, two 1GB sticks DDR 400, HD-3000 HDTV card, PVR-150 card, Iguanaworks RS-232 IR receiver/transmitter, Pioneer DVR-110 DVD burner