Hi,

Thanks for putting where you are located in your profile. Pretty area and rugged.

The AMM is suppose to adjust mixture for altitude changes automatically. The fact a pressure change goes along with your driving variable, is a clue. More so than a faulty AMM or coolant temperature sensor on a car that runs normally down below.

The higher you go up in the atmosphere air becomes lacking density.
What makes up air, to be what air needs to be, is now is deficient of of its 20% oxygen content, an oxidizer, that make gas burn in the first place.
By volume or weight there, this less consequently is the weight creating pressure, on everything considered to be normal.
As Ted says, it equates to 70% less power. This percentage, I did not know as a fact, but it’s a grunch!

A gas tank is a confining space designed to be a liquid and vapor holding vessel.
It is not allowed to be operated under a vacuum condition. This is due to it would promote vaporaization or a lower boiling flash point in gasoline.

Having any vacuum holding situation, would make it Very hard to continue to push gas out of tank, so it may not want to give it up easy. The engine may be getting fed through a straw!

You are not saying you are getting black smoke, just lower power.
This brings me to think that you might want to check the fuel tank cap to see if it venting or in this case letting air into the tank. That is until you go back down the hill!
It can be sticking or has failed shut. Mostly from the lack of operation. Cooler climates have less barometric pressure changes over time and/or are not very rapid.

I would either the crack the cap opena bit, when up there or get braver and suck on the screw end. You will or should see holes there. The air goes under the handle end and down the center.
Do a lot of fast intakes to shake up the flap as it might be slightly dirty or gone stiff. It should buzz a little bit but easily.

The cap is suppose to keep vapors from leaking out except in moderate to extreme heat environments.
I am not an emissions technician, but I imagine, when the cap does get checked, it is probably only for a pressure holding test. The tank gets pressurized all the way out to the carbon canister. This check all the hoses involved.

Worth a suck or shot, the next time you go somewhere, to get high! (-:)

Phil