I have had the same issue since I got my car at the beginning of July. Like another poster described, if I leave the car for a while, no trouble. It is mainly when I have popped to the shops and left the car for no more than about 5 minutes.
I note that when I get in the car normally, the screen is black until I press the start button, then the BMW logo appears and all is well. When it goes wrong, it is already showing "NO SIGNAL" before I have even sat down.
I have also found that you can clear it, by getting out of the car and locking up. Wait a couple of mins and then get back in and it is fine.
Pain in the arse though because even for short journeys, which in my case is having popped to the local small tescos, there is no reversing screen which is useful in those tight car parks.