I have problems with my iDrive as well. Last week it displayed "NO SIGNAL" in orange letters and required switching off to clear the error and restart the navigation. This happened 5 times during a 50minute commute. Then it was ok for a week but today the error occurred again (just once) and then later the navigation froze and the system reset itself. I have the Professional navigation.
I also have another issue in that no matter what song was last played from the music collection it always resumes playback from one certain song.
Will be going to the dealer when I get chance.