I see that someone earlier in the thread mentioned the pressure cap on the coolant reservoir. Did you change the cap? If so, what was the result? I ask this because I just attended to a few simple things in the cooling system of my 94 C280 in May, and I have found an improvement in how the car handles high heat load. I have always changed the coolant according to Mercedes specs, and always use the Mercedes coolant. But this time I changed every hose in the system (I don't trust 13 year old rubber) and had a new pressure cap installed. I've had the car for over 6 years, and have never changed the pressure cap. I have no reason to believe the previous owner changed it, either.

Now I'm not naive enough to think that changing the hoses has done anything for cooling system performance. But as I said, the car handles high heat load better now. It doesn't heat up like it used to when stuck in traffic on the highway, or when moving into city traffic after being on the highway for a while. I suspect that the cap was no longer providing its full pressure, which lowered the boiling point of the coolant, which then resulted in decreased system performance.

This might not be your whole problem. But it might be a contributing factor. I'd suggest replacing it and seeing what happens before you go pulling the radiator. I was very surprised at the measurable difference from this simple thing.