Posting an update to the intermittent overheat problem...

Basically after thinking about it for a while, decided that it had to be electrical. The only weak link seems to be the high speed fan relay. Did a swap with a known good one and the car immediately started to run cooler. Frankly, I'm not sure how the high temp aux circuit works. The coolant sensor is a NCT type and the black relay is a 40A. BTW The low speed silver relay is a 30A.

On opening it up, the 40A relay has a resistor built in for the solenoid action. I resoldered all joints and sanded the 2 (?) contact points. The contacts are also a puzzle, 2 contacts, one larger than the other. The smaller one contacts earlier then the bigger one. I studied the wiring diagram but can't make out if the signal to trigger the relay is a on/off type or by a variable voltage (from the NCT sensor) . If it is the latter, then the relay coming on and off times can vary according to the above mentioned resistor. Also I tested the relays with a transformer and the 40A seems to come on at a lower voltage.

Just thinking out loud if anyone want to add on....

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1992 124 E200 (Euro)