Tis the season for overheating..Help!

[glenmore]

I had a sudden overheating in my 1991 300CE, W124, M104 today.
I've had the car a little over a year, about 122k miles. Prior to these past two warm months
the car has stayed 83-90C. Lately with the warm weather and the a/c on and idling in slow
traffic it would creep up to 100C but fall back to 85-90C when traffic picked up. The two aux
fans would only occasionally come on and I would never hear the roar of the main fan.
Coolant level is as it should be. Today on the frwy, it went slowly from 85C right to 120C
before I pulled over. I limped halfway home, pausing to let the car cool. Having the heater
full blast helped some. Both aux fans are on now. The main fan spins along with the idling
engine but again I have never heard it roar full blast. The viscous fan appears to be in good
shape. It will not freewheel when the engine is off. When you turn off the engine, the fan
will almost immediately come to a stop. The action of the fan is not altogether consistent.
Sometimes it will spin 180 degrees, sometimes only a few blades when the motor is off. The
top radiator hose is hot. The air exiting the rear of the radiator is cool though. To get the car
home I let it cool from 7pm till 11:30pm. Driving about 2-3 miles home, the temp went straight
from cold to 110C and only the heater slowed the rise to 120C. Again, with the motor very hot,
the air from the back of the radiator seemed cool. The radiator looks somewhat new. Relatively
clean. I don't think it is original. Any ideas?

TIA

glenmore
1991 300CE