After trying fuel injector cleaners, spark plugs and wires, I took my car to Steve who diagnosed the problem! I did not wanted to buy a new fuel di$tributor. So, I bought a used one from e-bay and installed it myself. No idle problems now.
In other words, if you already tried to fix the rough idle with the "usual stuff", get a good diagnosis from a trusted mechanic and then look at your options.
Good luck!

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J.H.
'86 300E