Once more into the void...- does someone have a flashlight?

[Fuzzzydog]

Hello all, been away for a while since I picked up a 4X4 and returned to my
offroading passion, but the 88 5KTQ avant has lured me back.  Unfortunately
it is a love/hate relationship.....:(

It is a sweet car - for the most part been running very well since I've
owned it.  But recently I have developed an intermittent high idle that is
defying all efforts to fix.  I have read SJM and other sites and done a lot
of checking.  Right now I am considering different types of rope for a noose
to put myself out of my misery.  Before I go to that extreme, I am hoping
someone can point out what I have missed....

Symptoms:

MC engine, turbo, 5 speed, all stock - no mods.

starts up very well, idle a tad high but slowly starts fluctuating and
getting higher as it warms up.  go for a drive and now it won't come off
high idle - ie coming to a stop, push in the clutch and rpm stay at about
3k.  sometimes, only sometimes, I can let the clutch out to bring the rpm
down and they will stay down right around 800. this is rare though - usually
I sit there at the stop light with the engine doing 3k rpm.
car does not stall when oil cap is removed - even when idle is ok
always, ALWAYS, filled with Chevron super supreme - problem has persisted
over many fill ups
high speed is not affected - done a consistent 140km/hr trip recently -
pulls like a freight train.
oh yeah - this might be important -  when the high idle problem is there,
and I am coasting along at any speed, if I let my foot right off the gas it
keeps motoring along (because of the high idle) BUT, as soon as I give it
some gas to speed up there is a 'dead spot' right off idle.  The car bucks a
bit as if fuel or spark were shut off, then it picks right up and away we
go.  this dead spot is not there when the high idle problem is not present
(remember, the problem is intermittent)

things I have done.
checked idle switch - opened, resoldered, tested - all good
replaced ISV with one that came out of a properly working car
checked for broken vac hoses - all good that I could see
checked idle controller under dash and saw that 1 wire was broken completely
apart - fixed - no love - didn't cure the problem
battery is new
MFTS is brand new 3 wire
swapped the small single wire temp sensor at the top front of the engine.
checked wiring/hose connections - nothing stood out.

Willing to listen to any and all suggestions
I have lots of spare parts that I can swap out from a previously good
running 5ktq sedan.

Thanks for reading and looking forward to your input.