When doing very complete flush remove heat exchanger hoses, and use your garden hose to moderately power flush, both directions. And watch the crude blow out. The exchanger can like a settling pond , filling with crude . When flushing , block drains should be removed also, to flush low areas in block. Is top radiator tubes filling with crude reducing flow? Pull thermostat, in pan of water boil it , at what temp does it open? (Physical test of thermostat.) Test radiator cap, hold pressure, seal is good. That's a start.