I have a 1998 Z3 2.8 coupe. When I changed gear and released the throttle, the revs would stay up where they were for a while, and when they did fall, it was all of a sudden with a bit of a 'jerk'. The revs sometimes climbed up by themselves when I was sitting in traffic. I was told that it was a faulty potentiometer. I changed it, but the error was still there. My mechanic took it to the electronic test, but found no errors. But he did reset the car's computer, and ever since it has been fine. So I could have saved installing a new potentiometer.