I know i have a different year, but my 99' 528 had a very simular issue. When Idle at a light it would idle extremely rough and dip down alot. It got so bad I had to shut the car off and start it agaiN. I would also get a lack of power very frequently. As for the rough idle, it was a huge tear in the intake boot connected between the MAFS and the throttle body intake boot. The huge lack in power that prevented me from going 25+ mph was due to engine misfires in the cylinders. I had to change my ignition coils . Did you change the ignition coils, spark plugs, intake boot ?
I spend at least 2 hours a day cleaning my car
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