Sounds like exactly what I'm experiencing. I had to disconnect the battery for some servicing and my radio only stays on for minutes at a time. Is this due to some programming? I have a 2001 E38 but I think I have the Mk 3 where the LCD flips down to reveal the cassette drive. Should mine be doing this too?
What do you mean with the indicator on the unit being off before disconnecting the battery? I'm sure I didn't check this. I thinking that my Navigation unit is bad. Maybe it's just that I disconnected the battery and don't know how to reboot?