72-76C isn't the normal operating temp range on a M103. It's too low. If the correct 87C thermostat is installed, it may be stuck open, not allowing the motor to reach optimum operating temp. Or it's not the correct thermo. It's also possible you're getting faulty resistance readings from temp sensors. You may already know this, but the main coolant temp sensor is actually two identical sensors contained in a single assembly. One sensor sends a resistance reading to the EZL controller and the other sensor sends a reading to the CIS controller. Those signals are supposed to be identical readings, otherwise the two control units are getting conflicting information and attempting to make counteracting adjustments to the motor. It's easy to check the sensors if you have a multimeter. The temp/resistance table is listed in the FSM. There's also an air temp sensor located in the air intake cowl that can be checked and uses the same table of specs as the coolant temp sensor.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't