What light fluctuates? Headlights?

With a poor idle there should still be no change in light output unless the poor idle is a near stall. You may want to check the voltage regular. You do not have to remove the alt to do so, making it a quick thing to check. But a simple voltage check at battery can also hint at any issues.

If voltage is fluctuating greatly, it can affect idle. If you are concerned about spark plugs then maybe you want to change them, but they should not affect idle unless they are way past due for changing and have some heavy deposits. But when this happens there is usually more of a miss than an irregular idle.

Really, there are a bunch of things to check. Lambda adjust and vacuum leaks to fuel filter and air cleaner. Can you provide an idea of what has been done to the car?

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car