Q: I live at 9,200 ft. in the mountains above Colorado Springs. As long as I drive my '09 Forrester X limited at this altitude all is

I live at 9,200 ft. in the mountains above Colorado Springs. As long as I drive my '09 Forrester X limited at this altitude all is well, car runs fine, plenty of power, good mileage, etc. If I drive the car to Colorado Springs at about 6,500 ft., It almost always sets the P 0420 Catalytic insufficiency code. Had it drilled and pressure tested, no blockage. Did a injector/throttle body clean and ran premium fuel for several weeks with no change. I can remove the code and drive it as much as I want till I go down the hill again where it will reset the code. Took it to dealer who pulled the code and said it needs $1,200 new converter. As a side note the car was driven in the Springs for awhile before we got it with no problems till we brought it up here. So the problem seems definitely related to altitude change. Has anyone heard of this and what is the problem?

My car has 132000 miles.
My car has an automatic transmission.