[Shadetree]

Thanks for sharing this. An older gentleman rebuilt my `35 engine and has since passed away. He had my newly rebuilt engine on a test stand with a 25 gallon or so barrel as a radiator and we could see the water being pumped openly into it. He said `Watch this' he pulled the throttle back to a slow idle. The water flow completely stopped. He said people like to see how slow these old flatheads will idle, but don't do it for a long period of time.
I most likely have the same issue you had but I have masked mine with an electric fan while idleling in hot weather. Like your car, as long as I am moving even in 90° weather it won't overheat.