Driveability issues on my slant.

I agree that it sounds like vapor lock, I had a 66 mustang 6cylinder that used to vapor lock all the time, aluminum foil wrapped loosely around the fuel line to act as a heat sink helped some, but didn't stop it completly
This is going to sound crazy but, next time it acts up, put some ice in a shop rag and tie it around the fuel line, near the carb if it stops doing it then it's defiantly vapor lock( once we drove from Fla to Ohio stopping every 50 miles to add ice, I've been trying to forget that adventure for 30 years now, but we got home)
as far as coolant flowing too fast to cool properly, I've been told that's why to always run a thermostat even in summer, it slows down the coolant flow so it cools enough
But all these are things old shade tree mechanics told this old shade tree mechanic, when he was a young man