I'm having this exact same problem with 8.4 drivers. It seems that if the image size differs when changing channels it messes up the picture. However going back to the MythTV main menu and back to LiveTV fixes it and you can now watch the previously messed up channel. I tried the 8.6 drivers, but I suffered from the image corruption (reported in another thread) and couldn't get any video running from Myth.
Also with some channels I will not get the totally messed up picture, but instead two identical TV pictures (one on the upper part of the screen and the other on the bottom). Setting the MythTV scan mode to "Progressive" does help with this and fixes the picture. If I record shows with Myth they are totally OK, so it seems to be a problem with GFX driver.