Back when I was in high school I had a '64 Valiant, 225, that developed an intermittent over heat issue.

Turned out that the factory fit plastic impeller was basically free on the pump shaft, but had a little grip, when it felt like messing with me, the impeller would shift just enough in the pump cavity to stop it, fan still turning happily, no belt squeal, but no circulation either.

This didn't happen all the time, but every once in a while I would look at the gauge and the needle would be heading for the high side.

I was so glad when they stopped using those damn plastic impellers ...