Seems to me that what you have is a low circulation proble at idle. You may want to play around with pulleys to speed up the water pump.
Something else to concider, You may have some combustion pressures entering the cooling system. The pressures may interfere with circulation. Either look for bubbles in the radiator neck while idleing or attach a radiator pressure gauge to monitor rapid pressure increases. Could be due to an overlooked crack in the cylinder wall or combustion chamber.
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With all of the modern advances in technology and medicine, there is still no cure for the common birthday - John Glenn
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