Not normal. Sounds like you're due for some cooling system service including a new thermostat. Check the archives on thoughts of MB brand coolant vs other brands of coolant.

To qualify, getting to 100 occassionally is not a problem. Coupled with staying below 80 when you've been driving a while suggests that the thermostat is not closing fully. Not a stretch to suggest that it's also not opening fully.

Sixto
91 300SE
87 300SDL
83 300SD