I can't say I've ever experienced that kind of behavior in my car. It is a little rough when cold but I don't think I've experienced that even when cold. I know you don't want to go to the dealer but they might be the only ones who can figure this out, if anyone. Perhaps there's an independent shop in your area that specializes in BMWs, or European cars?
Sorry, I can't be any more help but I've never heard of this before. Good luck.
I've been assimilated. 6MT/ZPP/18"/PS extended.