2002 Ford Focus High rev

Engine light doesn't come on. We took to a small transmission shop in which they kept for a two weeks. They drove it around to get it to repeat the high rev. They said it drove perfect, except one time it did the high rev. When my son drives it he says if he lets off the gas it will drop back down. All fluid should be new and clean since a year ago we had the diff. Assm replaced. We took to dealership and they said they would need to get a code to know what was wrong with it, but it doesnt give a code. It has been about a month with it not doing the high rev and poof it happened again.

Thanks for responding