My 87 740 Turbo has recently developed fluctuations where the idle is normal, smooth then becomes rough and exhaust becomes smoky, blackish. The idle speed drops and may stall. If the idle air valve is connected, it adds air to (usually) prevent the stall. It's as if the mixture richens spontaneously. It then smooths out to normal again, exhaust smoke disappears. The engine starts/restarts easily. I can find no ignition fault. The idle timing (about 20 deg btdc) does not change and plug waveforms show no misfires, spark dropouts. The plugs are uniformly blackish (can be due partly to a lot of idling/slow running while trying to diagnose it). The symptom also occurs above idle on the road with some hesitation, lack of throttle response. Adjustment screw on the air mass meter does not affect it. Most parts of the injection system have been replaced preventively in the past few years. The Bosch Jetronic control unit is original. The engine wiring harness was replaced years ago to correct biodegradable wiring and has worked fine for years. Voltages at the radio suppression relay and injectors are all at spec. The sensors under the intake manifold are all original. I've owned this car since new, overhauled the engine at 269k miles, ran flawlessly for 10k miles until this. Any ideas would be welcome, thank you.