Did the Onan certified tech do the adjustment correctly?
Assuming we are talking about a single air/con unit on a 4000w generator, it should be OK even at altitude. My best guess is the tech didn't do the adjustment correctly.
One simple trick you can try is to turn on the fan mode, let it settle in for a minute, then flip it over to the air/con mode. This way you don't have both the fan motor and compressor motor surging at the same time...but this only works if you are borderline. If the microwave by itself is causing it to choke, it's probably not just close.
