What blend of coolant?
With some of the military equipment I worked on, we experienced overheating if the coolant had pure antifreeze
Second: Have you pressure tested the cap and coolant system?
for every pound of pressure created in the cooling system, the boiling point of the water is raised by three degrees. A 15 PSI cap boils at 257 deg versus 212, etc. This does not factor the benifit of glycol, nor the detriment of pure or too much glycol
Are there bubbles forming in radiator? Does it push coolant out (when cool) running?