W124 1995 280 E - 104 petrol engine idling at 1800 revs !!!!

This problem initially started out as the car would not idle at all. It would run perfectly but would die on idle. - engine hot or cold.
Fitted a s/hand tested OVP - no change. Vacuum leak none found (smoke test).
Took to Mercedes main dealer for diagnostic, over and above the 16 pin socket on the car. (they felt they could get enhanced data beyond the 16 pin socket.)
They indicated they could get no data at all from the ECU -cleaned the throttle body (presumably the butterfly in situ) they confirmed continuity on the wiring from the socket to the ECU and suggested a faulty ECU.
They fitted a brand new OVP relay and the car now runs with idle revs of 1800 warm and 2000 cold !! they were at a loss on this and could not suggest anything but perhaps investigation of ECU.
Would brittle wiring on the throttle body's wiring/soldered joints produce such a condition ?
Does anyone have any thoughts - I am at a loss - the car drives perfectly apart from the idle problem.

 

I think you will find your EHA is no longer working as it should ,.