Hello,  I have a 1999 Z3 coupe with a 2.8L.  When I first start the car,it runs perfectly.  As soon as it reaches operating temperature it has a low-rpm misfire (idle to 2500rpm).  It's quite noticeable in neutral revving it up, but is extreme when trying to drive the car.  Once you get above the 2500rpm,it smooths out a little, allowing you to apply more throttle.  The engine light will come on, but not always.  I scanned it once and it said the crank sensor was bad.  I replaced it and got no results.  Do you have any suggestions as to what to try next.   Thank you in advance.  

sounds like an 02 sensor. are you using a bmw scan tool?