120C isn't really super hot, though it's hot enough. What color is the coolant? Do all the fans work?

Sounds more like a head issue. Do you get hot air out of the climate control system? When you crank it to defrost, does the temp drop? Is there coolant under the carpet @ the footwells on either side of the dash?

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.