Is this condition with or without the air conditioning running?

If without A/C, then I'd say you have a cooling problem.

If with A/C, then it may be your auxiliary fan, fan clutch, or some restriction in the coolant flow. But you can generally expect temps to creep up in traffic with the A/C on full blast. Under such conditions 100C is nothing to worry about.

110-120 C is where you get really, really, deeply concerned.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles