I did some forum searching and it appears there was a problem with MythTV locking up from within LMCE after a few minutes. As far as I can find, no solution was posted and there hasn't been any talk about it since around December.
I'm have a DirecTV STB outputting to the S-Video port on a PVR-150 card. I can watch TV, change channels, etc. However, after a short period of time, maybe as long as a few minutes, it freezes. At some point before the freeze, the sound stops. I can ctrl-alt-f2 and login fine. alt-tab shows the lmce windows, but none of them actually get me out of MythTV. From what I've found, I can either reboot completely, or ctrl-alt-backspace to kill X, then restart X.
If someone wants me to try some things and help investigate, just let me know. If anyone has a solution, even better.