Doesn't the 2008 3.0 engine utilize Valvetronic for the throttle duties? If so, cleaning the throttle body does absolutely nothing. The conventional throttle body is a backup device in the event the Valvetronic system fails. Under normal use, the throttle body is held wide open at all times.

Incidentally, my sons 2007 328i does the random stalling thing. Haven't figured it out yet as it only happens about once every couple of weeks.