Baz is dead right over a/freeze ratio,I run a workshop that does loads of cyl heads and we see loads pitted due to a/f ratios being wrong or incorrect a/f being used,it also effects boiling point, ask any Spitfire pilot (glycol cooling system).:D

I would do things
Earlier opening thermostat with 2mm hole drilled in the plate to allow air to escape and flow a little water
Earlier starting fan switch