Very weird. You're either not getting sufficient coolant flow through the radiator (i.e. stuck thermostat or poorly performing water pump) or you're not getting sufficient airflow through the radiator (i.e. electric fan doesn't pull enough CFM through)

Does it run super hot at idle? You mentioned you have an electric fan. What kind is it? How is it mounted? If it's not shrouded, it may not pull enough air through the radiator.

Try removing the thermostat to see if that helps.