I just posted a very similar problem with my 1994 C280 with M104 engine.

http://www.peachparts.com/shopforum/showthread.php?t=128169

I noticed that my high-speed aux fans were coming on at irregular temperatures. Arthur Dalton suggested the culprit was the coolant temperature sensor and based on his recommendation I now have a new one. It was a $50 Canadian part, and about a 5 minute job with simple tools.

Based on what I've learned about the cooling system on this site, I'd be looking at aux fan operation. If the car cools down when you get moving again, then your coolant is clearly circulating properly. This should eliminate the water pump, radiator and thermostat as problem areas.

Jonathan