'41 won't crank when hot

[Joe Immler]

My 1941 starts with no more than 3 rotations. However, drive it for some miles and shut off. If the radiator temperature is above 180 degrees or so the starter won't crank the engine. Push the starter button and all I get is a clunk no cranking. Battery is charging fine. Starter and generator have been rebuilt.
Regulator and condenser check out fine. Open the hood and let it cool down and it fires right up. (?)