You guys should try flying behind a Gipsy Major. The manual states that in hot conditions the engine should be shutdown by - idling at more than 1000 rpm for over 2 minutes. rpm should then be reduced to a minimum, the magnetos earthed and the throttle smartly fully opened.
And it works. If the engine is hot and the weather too the Gipsy has a very nasty habit of 'running on', due to the rich idle mixture and hot spots in the combusiton chamber. The idle period helps to cool things, the minimum speed mag cut gives the best chance of a controlled shutdown and the open throttle ensures the maximum amount of cold air getting into the combustion chamber.
Having said all that it is pretty unusual to get these symptoms with an O-320 - but not impossible.
Cheers