I still think it's a bad earth contact -but something inside the car.
Sometimes connections brake as they warm up and cool down. These can be a pig to track down. Often unplugging and replugging will fix the problem but you'll need to get behind the dashboard (Not sure how easy that is in a Beemer)!
Something like that is worth ringing the main dealer - ask to speak to a mechanic and get some suggestions from him - chances are that it happened before on another car!