You and I are in the same boat. I have spent some time on irc talking to mandingo about myth and the same problems you are describing. There are some things mentioned in the forums before about always streaming from the backend that worked before but is nit helping now.
I too just reinstalled from the DVD hoping that the latest changes to make a preferances file for the myth update would be applied, but while running the createtbz needed fir the mds I wad informed that the versions were different. I created the file manually and am running the createTBZ program right now.
So while I do not have any solutions for you currently, if I could offer 2 things- 1) you are not alone with thus issue and 2) the developers are incredibly helpful and concerned about the users. As thom has said many times, the goal is to have this be an appliance that just works. Just have some patience and if you have some time to get on irc to offer your setup to help diagnose the problem so a solution can be found... It has been my opinion as Ive played with lmce for the last 2.5 yrs or so that while their have been headaches, the potential and functionality that I have had working far out weigh the headaches.
I will continue to work to get mine working and if something is found, I will be sure to post.