What is your engine temperature at highway speeds? If it is around 85-90, then your basic cooling system is fine and either the main cooling fan or the Aux fans are causing the problem. 100 C is normal.

According to MB, temperatures approaching 120 C are no cause for concern. High speed Aux fans come on at 107 C. You can change the resistance of the temp sensor to start the high speed fans at around 100 C.

The temp needle in your MB is very sensitive, thats why it moves around. That is a good thing. Other makes have a range of engine temperatures for a single spot on the gage. So, instead of slowly going up in temperature, the needle jumps very quickly when the car overheats.

__________________
Zafar
94 E320 58000 Miles