I would suspect the fan clutch. a. it is a common replacement item; b. it makes sense because in stop and go driving, that's the only way that air gets moved through the radiator.

I would expect a thermostat or radiator issue to cause higher temps under other conditions as well.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe