Hi I also have a 328is and have had it for 2 years! My car does not have these problems but I have learned a lot about them from doing mods and trial an error on my car. The bad idle and stalling could be from a vacuum leak, a bad Idle Control Valve (ICV) or as the other person said MAF. It is most likely a bad ICV though. These can be fixed quite simply but getting to them is the hard part! Unless you have already done it I suggest taking it to a good mechanic as it involves removing the throttle body, maf, injectors, intake manifold and then a bunch of vacuum lines etc, it is not a easy job! The way to fix the ICV is to spray high pressure air into it to get the gunk out. IF there is no gunk then you will need a new one!
For a vacuum leak listen to any hissing noises coming from anywhere on your car. Have somebody start and turn off the car as you stand listening under the hood. You will hear a hissing noise eaier with teh car off and the noise will slowly go away.
If it's the MAF then DO NOT get overchargerd liek the other poster. $600 for a MAF and labor is NUTS, that's how much I'd pay for a clutch install lol. THe MAF can be replaced in 5 minutes with only a flat head screwdriver!
Hope this helps you and good luck!
<a href='http://www.cardomain.com/memberpage/237652' target='_blank'><img src='http://img170.exs.cx/img170/5774/sig2uq.jpg' border='0' alt='user posted image' /></a>
Mods: M50 intake manifold, BFP CAI, Fan delete, CDV delete, Clears all around, M contours, Clutch stop, Debaffled exhaust, CF BMW emblems