I am finding this thread to be frustrating and confusing. ...
There seems to be no set processes as to trouble shooting this issue....
You chop and change your thinking and i have not seen any test and result with clear answers.?
Did you do the coke bottle test with the cooling system.? I may have missed it if you did.
Also from my experience with that stuff you you put into your engine is it does increase engine temps and causes restrictions in your cooling system.
Has this car ever had a major overhaul of its cooling sytem..eg radiator out and serviced properly. . New hoses and thermostat. ?
What happened to the guy you said you had coming to look at it or did i miss read this.?