While it really sounds like a vacuum leak to me, I would consider replacing the idle air control valve. That's the silver canister under the intake manifold with 2 3/4-inch rubber hoses and a plug going to it.
Get one from any 85-88 240 or NA 740. Ought to be a good junkyard item for $10-20. I'd just plug it in and try it, see if that makes any difference.

If not, I'd definitely go over that intake again, spraying WD40 or carb cleaner onto all the seals. See if anything makes the idle change that way. If so, that's a leak.

Good luck!
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Rob Bareiss, New London CT ::: Roterande Fläkt Och Drivremmar!