Maybe things aren’t so bleak after all…
I went ahead and upgraded to Kubuntu 7 (Feisty), and it seems the new version of the X server no longer hangs with the version of the nVidia module I need for working TV-out support. And the crash when MythTV stopped playing was related to the GLX module not working. The GLX and Composite extensions can’t both be enabled at the same time, and Composite is enabled by default with the latest servers. But the Composite extension doesn’t work with this ‘legacy’ nVidia driver anyway, so disabling it allowed GLX to be enabled again, and then MythTV could use OpenGL properly and no longer crashes.
Next up was the sound card ordering problem. I have two sound cards in the system, an SBLive for regular system sounds, and the built-in motherboard audio, used to drive the TV. Kubuntu was detecting them in random order though, so sometimes the SBLive would be considered card 0 and the motherboard card 1, and sometimes vice versa. Every time I rebooted and started MythTV, I wasn’t sure whether the show’s audio was going to come out of the TV or the computer speakers, and if it was the wrong one, I’d have to go back into the config screens and reassign the audio device.
Fortunately, I found a page which describes how to permanently assign the sound card order, among other sound problems. Now the SBLive is always /dev/dsp and the motherboard is always /dev/dsp1.
And with that, all of the major remaining migration problems are now solved. Now I can get around to catching up on the seven weeks of recorded Lost episodes I still haven’t watched yet…
Update: Nope, the X server continued to freeze on me, it just took a little longer this time. It looks like disabling AGP entirely works, but then X takes up a lot more CPU time. Oh well, as long as it works…