couple things you may not have thought of.why should you believe the temp gauge?you spoke of the crusty stuff in the radiator what if this is deposited on the temp sensor for the gauge and also on the 212 switch?did changing the thermostat help any?i have seen those old stats only partially open.the only things that water pumps do wrong is to leak i have never seen one that did not pump.if the fan clutch is not locking up it will have an idle issue not high speed.it is possible that the aux fan is running backwards and cancelling the airflow accross the radiator.the aux fan has two sensors [1]at the drier for low speed brought on by the a/c [2] 212 switch as emergency cooling [high speed].the relays for this are in the fuse box and the low speed[a/c] sends the power thru a large resistor to drop the voltage and so the speed.when i owned a 77 450sel in texas it was normal to run 3/4 of temp gauge with a/c on.good luck david poole european performance dallas tx.