As you go higher, you run richer because the air thins out, so the odds are you're going to either need to tweak your A/F ratio or rejet. Depends on your setup right now and what your tuning elevation is set for currently.
Give this a shot.
http://www.thesamba.com/vw/forum/viewto ... t=altitude
How are you jetted right now? How is your engine set up? Give us a rundown, as the parts inside will also make a difference in how your jetting should be set.
There's a lot more to it than just changing the idles. The adjustments required will be different for every setup. There's a lot of plug reading and jet tuning inolved and if you're not accustomed to doing it, you'll probably have to make several jet adjustments to get it right.