I have 1990 m3 with 97,000 miles. After it has run for about 15 minutes, it becomes very contrary at lower rmps. (up to about 2700 rpm). I have rough idle and the car jumps and hesitates during even mild acceleration. The a/c seems to aggravate the problem. I might think the car needs a tune up, but I just took it to the dealer for a stage II inspection. Could this be an O2 sensor problem? BTW the problem started several weeks after the stage II inspection which included things like a valve adjustment and spark plug replacement.