My 1988 420sel has just developed a problem where when I am accelerating at lower speeds the engine seems to sputter and surge. High speeds are not a problem but it seems to hesitate and surge all the time during acceleration. The spark plugs and wires are all new, car has been fully serviced and tuned up. Starts right up, idles at 1000rpm when parked 7500 when in gear. Any ideas? Thanks