Had this issue on my 81 300CD 2 years ago. 100 c in slow LA traffic. 85 c at 65 mph. I found the fan clutch never fully engaged. Once changed the temperature stabilized at 80 while driven and went to 90 on a hot day with A/C on. Well within spec. My 81 has no coolant recovery system so its very important to have the temperature stable. Also nobody ever checks their radiator cap. If it releases early, temperature will be higher and possibly erratic. I've always gone by the rule that stable temperature is more important than the temperature itself. Any engine will be more efficient at a stable 90 c than one that fluctuates between 80 and 100 c. I read alot of posts where the person is trying to lower engine temp when in fact, closer to 85, 90 c and stable is the preferred goal. Also, make sure that the A/C condenser is not plugged up with debris restricting air flow. A garden hose aimed at the engine side of the radiator will be the best way to force the crud out.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.