Have a read through this
http://www.e90post.com/forums/showthread.php?t=136258
In summary, well documented problem with BMW 2.0L 4cylinder petrol engines(and some 6 cylinder petrol) especially those with Efficient Dynamics auto start and stop technology-built dates post September 2007. Main problems faulty coils, injectors. Remedy?-replacement of coils, injectors, spark plugs and software update.
Is this a long term fix? - no one knows some people end up replacing all 4 coils, injectors and plugs and yet the same problem reccurs.
Hope this helps. Oh..worth getting BMW's very own extended warranty if the car is out of manufacturer warranty as there seem to be a reccuring theme for this problem..just my two pence