[Brian]

I'm thinking you have compression leaking into coolant. Have you retorqued heads? Your other thread about rough running could well be a symptom of compression leak[s]. These cylinder heads need to be retorqued after heating/cooling cycles until they hold torque, as the gaskets continue to 'settle'. There is a way to check coolant for byproducts of combustion....
I maintain that a fan is only neccessary for idling and low speeds, once airflow is passing through radiator, courteousy of moving down the road, the fan becomes redundant.