1994 C180 Elegance Auto erratic idle/ throttle surges

[shalligan]

My wife's beloved C180 has been in her owner-ship since 1997.

Having spent the last decade eradicating miryad faults with this tribute to German engin-erring, there is one persistant fault which has so far evaded detection.

On throttling off, the revs pick up and hover around 1500 rpm, surging three times before dropping. The revs drop upon dabbing the brake, but still surges at idle, which can be a menace, especially at junctions.

I have suggested she buy herself a proper Merc.

Any suggestions?