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Hi Adrian, I'm Clayso a certified master automobile technician with 30 years of automotive repair and diagnostic experience.
From your description there is a flow problem most likely the radiator itself. What is happening at highway speeds is the coolant in the radiator is not cooling down enough fast enough to keep the engine temperature in the normal range. Simple drain and fills and radiator flushes will clean out the tanks of the radiator but these procedures don't do a very good job of cleaning out the tubes that connect the tanks of the radiator. The tubed or finned area of the radiator is where the actual heat exchange takes place, when the flow becomes restricted through all of the tubes the cooling system just can't cool down enough before the thermostat opens up again and lets overheated coolant from the radiator back into the engine. Replacing the radiator is one sure way to solve flow problems. This system if not properly maintained will gel up and plug the radiator.
Another thing to check is the water pump. If the water pump is bad, or the pumps impeller is loose the heater will not heat properly.
I hope this information is helpful to you. Should you have any questions please let me know.Kindly rate my assistance in a positive manner so I may have credit for helping you.Positive ratings are greatly appreciated.Thanks so much.