my 99 328isport, when at idle will sometimes drop 400rpms and almost stall, I have an aftermarket cone style filter instead of hte air box, other than that i t is stock. I just put new plugs, fuel filter, and cleaned air filter,. It does it more when it is warming up. usually halfway up to temp, but it does it hot also, and cold, no real ryhm or reason to it, any one have this problem, ...any suggestions, .. . o2 sensor , vanos, mass airflow, ...

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

Need A new Vanos Unit
Sorry to be the one to tell you had the same issue they tried software which fixed it for about a month. Ended up that I needed new unit. The good news is if you reported this to the dealer and they didn't get it right the first time. BMW will fix it even if the car is outo f warranty if you call BMW/NA Call 1-800-831-1117 when prompted say 8559 that is the extension you want to be transfered to. The young lady who anwers the phone will take your infomation and find a dealer in your area to do the repair. Let her know that you need a new Vanos unit due to cold idle problem.