Idle and decel strange behavior

[scott.john.mockry@bangate1.TEK.COM]

Reid,

Sounds like a defective or intermittent idle/full throttle switch 
(black box) which is located on the throttle valve shaft.
The deceleration fuel shut off valve is controlled by
the ECU depending on the signal from this idle/full throttle
 switch and what the engine RPM is. If the switch
is intermittent the car will surge during deceleration.
You could also have a sticking deceleration valve
as I noticed a service bulletin pointed out this problem on
some 200TQ's

My 89 200TQ had the same deceleration symptoms 
and intermittent high idle speed you
mentioned until I took the throttle switch apart
and resoldered the cracked internal connections.
My previous 86 5000T had the exact same problem,
as did a friends 5000TQ. I bet there are a LOT of
5000TQ's and 200TQ's running around with intermittent
idle/full throttle switches as this is a common failure.

HTH
Scott M.