let see you flushed the system installed new thermostat,how old is the radiator is it original no repairs ever done twenty something years old,and from my own experience I would say you have a stoped up radiator find you a new one,get one brand new for a 150.00 , as far as the water pump either its working or its not never heard of impellar on water pump wearing down if it,s loose or leaking you replace it. I have seen many a radiator over looked when it was the problem, if you presure test the system and you have no leaks system is not leaking down it is not the engine. now for the fan clutch, when engine is cold it is loose when it is hot it is working and forces more air and is hard to turn by hand. Jerry