[fanaudical]

Am agreeing with a tuning issue or vacuum leaks - I've had a couple CIS cars and haven't experienced any noticeable issues with elevation change below ~5000ft. (Added - None of my CIS cars had O2 sensors or altitude compensation.)

I'd start with checking that the pop-off valve seals (if you have one), then look at hoses, injector seals, air box seams, etc.

Another thing to check is to make sure your vacuum-actuated decel valve (if you have one) isn't open at idle at lower elevations and providing extra idle air (it shouldn't be - might need adjustment).