First, test the opening temperature of your thermostat on the stove in a pan of water. It should begin to open at 87 deg. C and be almost fully open by boiling. If it fails this, replace. If not, then the likely culprit is the radiator. When was your radiator replaced? It sounds like the insides of the tubes are crusted with deposits since you say the fan clutch and radiator core don't get hot. Also, with the A/C off and the aux fans running is the air that exits the radiator hot? Again, if the thermostat is good and the air exiting the radiator is not hot suspect a dirty radiator. You could try to flush and clean the system yourself, send the radiator out to a shop to be professionally cleaned, or if the radiator is original, replace it. You will still need to flush the system but IMO even an 8 yr. old plastic MB radiator is living on borrowed time. Mark