Hey,
I'm not a BMW pro or anything (only on my fourth one), but every time I've had that issue, starting with my old 320i, my 318i, and now my 735iL, it's usually come down to a loose or old vacuum line.
As a general rule, when I buy a bimmer, I replace all of the hoses I can. It's cheap enough and easy enough for me to do myself.
Just my .02
PS: STAY AWAY FROM THE DEALER!!! Unless of course, you are independently wealthy and can afford to pay three times the going rate for repairs. I've found several smaller BMW/Mercedes service centers who are trained and do a wonderful job.