[nize]

i used to have this same problem for years and finally installed the lindsey steam vent hose and the problem went away.

one issue is that a pocket of air gets trapped at the very back of the engine head by the firewall which can never be removed, and the kit returns that air back to the coolant reservoir.

it requires drilling a hole in the engine head and the best way is to remove the head to do this right. the kit is here:
https://www.lindseyracing.com/LR/Parts/STEAMVENT.html