High altitude driving issues

With a normally aspirated engine (no turbo or super charger) you will be loosing something like 10% of your horse power for each thousand feet you go above sea level. If you have a turbo diesel or turbo charged gasser, this power loss is almost non existent. Also keep in mind that water (also coolant) will boil at a MUCH lower temperature at high altitude so you MAY have lost some water from your radiator. Check to be certain you have a GOOD radiator cap on your engine to keep the coolant under correct operating pressure.

How long since you cleaned the "backside" of your radiator stack? Ours started doing that and as soon as I cleaned the crud of the backside of the radiator and allowed it to breathe again. the temps immediately went down at altitude.