Why is the coolant not circulating

2009 BT50 3lt turbo diesel.
About 2.5yrs ago it overheated, was told it had a crack in the radiator, had the radiator replaced. Continued to overheat, found sealent had been used, diagnosed with a cracked head, so had the head replaced.
I have since driven around 45,000km and it overheated, it would overheat when driving up hills and cool down when driving down hills. Temp would drop when going over rough bumps. Some trips it will boil and others it will not. Had the thermostat, water pump, replaced, flushed backwards to clear any airlocks. Had the the cooling system pressure cleaned and tested, was told it is all working fine. Still boiling – it’s like the coolant is not circulating properly. The bottom radiator hose feel cold. When the heater stops working it seams to over heat. No evidence of oil in the coolant, it’s not blowing smoke, not running rough, starting fine, not going to limp mode. Totally out of ideas!!!