OK just read it all and to my surprise NOBODY has reported back that the idle Air Control Valve was checked to see if it's rest position at normal temps was within spec or anywhere near spec. IIRC it shouild be about 0.04" or 1 mm.
Just saying it 'works' when called on by the ICU/ECU to regulate idle isn't enough.
The base idle is set to regulate around a IAC valve position that is governed by a spring and bi-metallic element. This position fluctuates with age, and it opens wider when old, letting more air through the bypass loop.
The ICU can regulate around an old IAC to a point, but if the initial position is too wide open, the idle will surge at first then the ICU will control it down, and compensate but the spring will over compensate and raise the idle again. This may go on until there's a default reached where the control ceases.

The initial spring position of the IAC valve can be adjusted by clearing out the potting on the external adjustment screw, and the valve returned back to the spec position, but it CANNOT compesnsate for the error from the temperature dependant bi-metqalic element being worn, and will give erroneous position as temperatures change, like from cold start to warm-up, and as the IAC circuit struggles to recover the correct idle the speed will fluctuate or stay high.