Don,
Regarding the percentage of antifreeze, you can analyze the characteristics of ethylene glycol with data from the Engineering Toolbox - Ethylene Glycol Heat-Transfer Fluid to see why the label on the Prestone bottle says "Minimum 50% Prestone/50% Water" (for corrosion protection, etc); and "Maximum 70% Prestone/30% Water" for cases where you need extreme winter cold weather protection. The 50/50 mix provides the "happy medium" that is good for about 90% of the people. As you look at the tables in the Engineering Toolbox note how fast the dynamic viscosity increases with increased percentages of anti-freeze in the mix. The best heat transfer will be achieved with 100% water, but you will have no protection for dissimilar metals (galvanic corrosion), no lubrication for seals, and lowered boiling point with your pressurized system.
I predict that the shroud you installed, plus adding water to your 100% antifreeze fill, will solve your problems.