Could this be a bad temperature sensor?  Long drives in the mountains could raise the temp above normal range and draw attention to a problem that isn't noticed at normal temps.

Bill

            > We would sure appreciate any advice we can get on
            > the following problem.
            > Do you know of any sites where we could post this?
            > Thanks.
            >
            > Vehicle: 1987 Vanagon GL Syncro Adventurewagen
            > Camper
            >
            > I bought this vehicle new and have driven it 324,000
            > miles.
            > It has had this problem intermittently ever since it
            > was new.
            >
            > Description of Problem
            >
            > When climbing from the valley up to the mountains,
            > the engine starts running
            > rough and losing power. I have never been able to
            > bring the vehicle in for
            > service when this occurs, because it usually happens
            > on long trips and rarely
            > when driving in town. Here are some observations
            > about this problem:
            >
            > 1. It usually begins when you first start gaining
            > elevation, with the engine
            > cutting out intermittently, and gradually gets worse
            > if you go up to the
            > passes.
            >
            > 2. When you come back down to the valley, it
            > gradually gets better until it
            > is running smoothly again, usually when we get back
            > home at sea level.
            >
            > 3. If the vehicle is parked for a few hours in the
            > mountains, it sometimes
            > runs better for a while afterwards.
            >
            > 4. When it gets real bad, the engine will
            > occasionally backfire, especially
            > when decelerating. The idle speed becomes erratic
            > and it sometimes dies when
            > idling.
            >
            > Repairs Which Have Not Helped
            >
            > The vehicle now has 324,000 miles. It has always
            > been serviced at a dealer.
            > The following repairs made over the years have not
            > solved the problem:
            >
            > 1. Replaced engine twice due to burned valves, at
            > 246,549 and 298,622 miles.
            > Also replaced rotor, cap, and ignition wires at
            > 246,549 miles.
            >
            > 2. Replaced Engine Control Module (025-906-022-D) at
            > 285,273 miles because
            > the engine would not run for more than ten seconds.
            > I was told that the air
            > flow meter harness was also replaced at that time.
            >
            > 3. Replaced Idle Controller (251907393D) at 246,549
            > miles due to erratic idle.
            >
            > 4. Adjusted Throttle Valve Switch at 201,643 miles.
            >
            > 5. Performed all recommended service at recommended
            > intervals for 13 years.