what version are you running? 8.10, 10.04, dvd or netinstall? perhaps that kind of information will help.
as for me, I'm running 10.04 beta dvd-install..I too experience quite some instability with mythtv like you describe. After a few trials it finally starts.
I don't know if its related, but I'm also having an issue with booting an md, that it sets the drives in the core as offline see here: http://forum.linuxmce.org/index.php/topic,12770.0.html
after doing that, it seems to be a little less reluctant to start, but its not were it used to be.
I just wondered....how responsive is your system now? I did a fresh install with the dvd, because I probably had screwed something up earlier...but still...it seems to me that everything is just taking so much longer now.