Doesn't sound like the fan clutch is right, but, for sure, there is a thermostat problem if it goes down to 70ºC after warm-up. The thermo on that car is an 87ºC; shouldn't go below that after warm-up.

I'd also shoot it with an IR temp gun at the head, therm housing and radiator to make sure the numbers are being reported correctly by the gauge.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)