80/20 mix at high altitudes
I noticed that you said that you have an 80/20 mix of water/antifreeze. I seem to remember from chemistry that water boils at a lower temperature at higher altitudes (less pressure). So - maybe you're getting too much boiling from the weak mix that you've put in the car. The antifreeze raises the boiling point of the coolant mix - in addition to lowering the freezing point.
I know that more water makes for more efficient heat transfer - but perhaps your weak mixture is being counterproductive at high altitudes.
Just a thought.