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I am the proud owner of a 1986 240 sedan that had a similar problem.
Idle was fast at times and unsteady. Also the engine started running rich. (black smoke out the exhaust).
I checked obvious things that you have checked. Finally took it to a shop that is known to be savvy.
Problem found: Engine wiring harness. Apparently these were made with wiring insulation that breaks down over a period of time and then the wires begin to short here and there.
I remembered seeing something in a IPD newsletter about replacement harnesses and why they go out.
Ordered a replacement from IPD in Portland, Oregon USA and the shop installed it for me.
On the road again. This I assume happens often given the aftermarket availability of this harnesses.
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