If your car is a '90 and the radiator has never been replaced, you may want to do so...your car has a notable tendency to have its radiator fail catastrophically. Not to make you paranoid, but more often then any of us would like to admit the 300E radiator top necks fail-if you were to not notice the huge water loss and attendant temp increase, you could cook the motor. I just pro-actively replaced my 300TE-I figured the old one had gone 170k mi and 13 years, so that was long enough: plastic gets brittle.
As to your case, it might not be a poor performing radiator; I know sometimes that a lack of a thermostat allows the coolant to flow through the radiator too quickly to cool properly, so perhaps that's why you still experienced elevated temps?
Good luck, and keep us posted.
"If everything seems under control, you're simply not going fast enough" --Mario Andretti
1995 E500 street car
1986 Porsche 944 Turbo S track car