2004 VW New Beetle power loss problem--urgent

So. My VW New Beetle has 99K miles on it (mostly highway–all driven by me). The car is manual and 2.0L and I’ve been very very good to it. I live in Arizona, where it’s ungodly hot in the summer and every summer I have a problem when I drive down from parts north (4500 ft), where it’s much cooler, to Phoenix (1000 ft). Toward the end of the 2 hour drive (at around 85 mph–with the a/c on most of the way and up and down a series of fairly steep hills–cars stopped on the side of the road steep–the car will begin to lose power and/or will stall. It has been to VW many times and not only can they not get the car to reproduce the problem, but they’ve checked many things, fuel pump etc. and the car throws no codes to indicate what’s wrong. The symptoms feel like old-fashioned vapor lock, or like some vacuum in the car is not sealed (maybe an o-ring swells in the heat, or something?). In any case, the car’s been doing this now for years, but only in the summer.

Just last week, though, when I was trying to get back up north, my car started bucking and threatening to stall on the way back up five-mile hill. Note, this hill’s incline is such that there’s a highway sign recommending that you turn off your a/c to avoid over-heating. I always do. It was the worst of the worst. And now I really need to figure out what the problem is.

Any ideas? MAF sensor? throttle body? the engine computer itself?? I’m desperate. It’s my only car and I have to go back and forth to Phoenix occasionally during the school year.

standard or automatic? Is there a possibility here that it is the transmission that is overheating and not an engine problem?

It’s a standard shift. I don’t know enough about cars to know if the transmission would overheat. Would it? On the highway while set on cruise? You have me intrigued!