IF the bottom of the radiator is cold, then doesn't that mean the the radiator is doing it's job? IF the temp gauge was reading ok, and it was boiling over, then I would say either the gauge went bad, or it is ok and you have air bubbles escaping, I am thinking Air in the system also. Or a water pump. My last shot would be a radiator at this point.

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83 300SD Dark Silver Dark brown inside