I went through months of mental turmoil over my 1987 300D's temps last summer. It did about the same thing ... would run around 90, go up to 100 sometimes, occasionally climb above 100.
Aside from finding out my temp gauge is 5 degrees too high, I have also found that it seems these cars are designed to run there. As you can see from other responses, a lot of folks' 603s hang out in that range. You just want to be sure that your t-stat works, you have plenty of coolant, your fan clutch engages and you aux fans kick in (should you reach 110). If all that is in order, as far as I know, 100 on climbs or in stop-n-go is not cause for worry.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)