[mr0077]

el, when you go to the radiator shop, ask them (if they don't suggest it) to test the coolant for combustion byproducts. If you have a slight head gasket leak you could get the volcano in your video, with a small amount of combustion gas leaking into the coolant. If so, the superhot combustion gas could add heat faster than the fan could take it away, and maybe at speed it has enough airflow to keep it cool(er)?
Just a thought...