1994 Ford E350 RV Class C

We have stumped our trusted mechanic and another one. RV drives beautifully until we have been climbing awhile over 6000 feet and for several hours. Engine then coughs, stutters, slows down and looses power. We shut it off, wait half and hour and then it starts and runs just fine until the next time. We have new fuel and air filters and new fuel pump. Mechanics have done all sorts of pressure tests. HELP what can it be. It feels a bit like the old fashioned vapor lock. Any clues???