1992 245 B200F M47
LH2.4 Jetronic

Problem: engine idles steadily and at the correct 750 rpm after every start, cold or warm, but usually shoots up to 950 rpm after use of the car. This is intermittent, but it usually idles at a steady 950 rpm. It usually occurs directly after driving off or may take some time. Engine temperature seems to have little or no influence.

Also when coasting, it idles at 1200 rpm dropping slowly to 1000 rpm and only drops down to 950 rpm after coming to a standstill.
At the moments when it does idle correctly, coasting results in a sharp drop to 750 rpm one or two seconds after releasing the gas pedal, coming to a standstill does not affect idle any further. Some people say it should coast at 1000 rpm only to drop to 750 rpm when coming to a standstill.

The usual suspects have been eliminated:
- new IAC,
- TPS functions reliably and is adjusted correctly
- TB cleaned and functioning correctly
- no vacuum leaks
- ECT within spec
- AMM not faulty
- VSS present
- no fault codes
- electrical connections cleaned
- ground connections on intake manifold and between head and firewall cleaned
- pcbs of both ECU and EZK inspected and through hole components resoldered
- electrical signals measured at ECU and EZK, all present and correct, apart from when engine idles high, then signal to IAC is set differently.

Basically it looks like the ECU/EZK decides - for whatever reason - to idle the engine 200 rpm higher most of the time.

I am running out of ideas and these particular symptoms don't show up in threads or FAQs.
Any suggestions what I might check next?

Your help is much appreciated.