No real offer of a cure from me, other than to say I have a similar situation with my 3 ltr manual.
With the original (freshly cleaned) Idle Valve installed, car starts from cold OK, sits circa 1200 revs for a few minutes (think this is normal) then settles to 500/600 revs idle when warm. Sometimes revs settle at 2000 for no reason, can be stationary/neutral or when the car is being driven. Depressing the clutch sees the revs rise. Similar effect to cruise control jamming on.
Fitted alternative (freshly cleaned) ICV from scrappies. No 'cold start' rise in reves, idle is 1000, no hunting or increase in revs any other time.
Swapping the ICV's around simply swaps the faults/characteristics. When the revs are being held high if I remove the electrical connector at the rear of the ICV then the revs fall and get a warning light on the dash. Reconnect, revs stay low/where they should be for a while, but then can rise. warning light goes out again.