My Bimmer's a bit older than yours but the problem sounds similar. I found out about this accidentally.
I was getting my oil changed and the guy was showing me something on one of my air intake hoses. He didn't get it back on right and that's exactly what he was warning me about. My car was idling fine before he touched it. When I started to leave I knew something was horribly wrong as the idle dropped so far down that I nearly stalled when I pushed the clutch in and it was shuddering. I had him put that hose back on properly and the idle was fine.
After that I checked this problem out on the internet and found that if there is even the slightest tiniest crack in any of the air hoses, it can cause what seems like an idling problem but what is really a vacuum leak.
Just a thought....good luck.
Juli