I am not sure if 3800 feet would be enough to cause the fluid to be visibly boiling. I have gone to 3600 feet on a spirited run out the Ortega highway several times, in 90 degree outside temperatures, and didn't loose any coolant when stopped at the top.

On the other hand, could your overflow tank have a bad cap or seal and be allowing the tank to depressurize?

------------------
JCE
87 300E, 65k miles
Smoke Silver