Have you filled it since you have been in Flagstaff?  We have lived in AZ
since 1973 -- 10 years of that in Flagstaff.  I have been told that gas is
formulated differently for high altitudes.  I know that people who drove up
to Flagstaff often experienced hard starting the next day.  We used to try
to be at 1/4 tank or less, and then fill it in Flagstaff first thing.  BTW,
Phoenix is at about 2500' of elevation.

>I just drove across the country in my 85 westy with no problems. Driving
>from Phoenix to Flagstaff in  a snowstorm my bus started running cold.
>Really cold. In Flagstaff the next day it still ran cold and a signifigant
>drop in power. When I drive around town awhile the temp comes up but it
>still can't get out of its own way. I just had the head-gasket done. It
>also idles rough when cold and stalls when it first starts. It idles
>poorly when warm too. Could the altitude from Phoenix (1000 feet) to
>Flagstaff (7000 feet) cause the problem? Thanks!

Bill
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Remember, it ain't what you don't know that hurts you,
        it's what you do know that's wrong.