Re: even with 8756, minimal multitasking slows down glxgears performance
could you please try a different tv application - like xdtv?
xdtv can record shows too, even schedule it, and it needs much, much less CPU. mythtv is a real CPU hog.
If your problem still exists with a different app, than we have to look elsewhere - but I don't have very fond memories about mythtv....