Another? random thought for you. The engine temp sensor is between the two cylinders. The wiring to it is in a harsh environment. On mine i found it resting in motor with some wear on the sheathing. If this sensor has an intermittent issue it could affect the idle? If it shows engine cold then ecu would up the revs.
Since all the obvious stuff doesnt work, panic and go improbable.
I cant remember if you have tried new intake seals, or checked inlet manifold for cracks but this has been a problematic area for a few.
Chin up, at least yours is mobile, mine has comprehensively broken the chassis in both common failure areas. Waiting for new chassis.
