[zellerj]

The intermittent over heat is probably caused by something floating in the cooling passages that gets in a position to block water flow. Turn off the engine and it floats out of the way and all is as it should be until you start up the engine again.

Cajun is right with the diagnoses.

I would bet on the exit to the exhaust manifold because the exhaust manifold is large enough that it can have large rust flakes that can swim around and periodically block the exist.

Another place to look is at the T going into the side plate directly after the pump. Bits of impellers of Christmas's past can get caught there.