1986 Other Mercedes Benz Models Engine Overheating. Or not

I have a 1986 MB 300e, good shape overall. I've had an overheating problem for a few months. Every time it's "fixed", temp is normal for a few weeks, but then tends to "overheat" - in quotes because the car doesn't have any overheating symptoms: coolant doesn't boil, but rises only to normal hot position on reservoir, airflow thriough radiator is warm, but not inappropriately hot, engine compartment is not overly hot, no burning of oil or any indication of coolant in oil (no real loss of coolant even over time.) Temp guage is only clue - the aux fan doesn't go into "disaster mode" (high speed), but the fan sensor and fan functions have been tested and work fine.

Up to now, I've replaced all hoses, water pump, radiator, reservoir (original had a leak), and have ordered and will replace fan clutch this week (I think it is not clutching in when it gets "hot", if this is really the case, and I've replaced everything else.) The thermostat was checked and was OK (and in good physical condition.) My mechanic, who is Mercedes certified and appears very capable, is a bit stumped.

What are we missing? Many thanks!