This is a 2WD '99 Nissan Pathfinder with a newly installed automatic transmission still under warranty. While cruising at steady highway speeds the engine will begin to rev higher without the car accelerating along with it. Never occurs while accelerating, just cruise speeds. The mechanic says no codes are showing - can’t figure it out. There is a burning smell, though. I suspect a slipping clutch somewhere but wouldn’t that happen while accelerating?
You need to put that warranty to use. The new transmission is junk it sounds like. A slipping transmission will not produce a code in many vehicles, so don’t let anyone use that as an excuse.
Is this happening on inclines? It is possible that it is the torque converter disengaging, but you should still take it in since it is under warranty.