My shop put in 3 new heads and sealed it all up brand new 3 months ago. New water pump. New head. New gaskets all around. New new new. Everything ran beautifully for 5 weeks. Then the first over heat but bad thermostat. I replaced the thermostat and all was resolved,,, for 4 weeks when it over heated but the thermostat worked and showed what was happening. This is when l found out about air getting trapped and purging. So now lve been trying to resolve it through purging air,
I agree that some of the behavior made me wonder about head gasket but.. it has full power, full pressure, no water oil mixing nothing else behaves head gasket leaking except the over heating itself.
Yes when l idle , it gets hot down under the engine. It stems from down below the engine and then the reservoir gets too hot and boils and steams pretty quickly. Even before the thermostat says so. The thermostat almost seems delayed.

It also seems like when l open the thermostat port it resolves it briefly.
Not permanently. This is why l kept thinking it was air.

So. Can l have a tiny head gasket leak that hides. ?
That doesnt draw power. Doesnt mix water and oil and takes a few days to draw air into the system before there is an issue?