Petes ES300h

Has anyone else experienced this? No replies yet - help! I don't want to take my car to Colorado again!

My 2015 ES300h is a great car, but i experience something very odd at high altitudes. This has happened twice - when driving at altitudes above 11000 feet, probably going downhill, my engine rpms suddenly start to rise without my car going any faster. When it passed 4000 rpm, i pulled over and stopped to get the engine back to normal. I would then begin driving and within about 20 seconds, the engine would do the same. This happened about 6 months apart. I've had it checked by two dealers and it shows nothing was wrong. Has anyone else had this happen? Any ideas what's causing it and how to make it stop?