1994 Ford Bronco Engine/fuel

While driving at high speeds (55 mph), I experience a high rev of the engine, like it is shifting out of gear and than goes back to normal running. Sometimes it will happen for a few seconds, than back to normal and than rev again. My transmission man says the transmission is not the problem. The transmission does have about 75,000 miles on the last rebuild. I would appreciates any insight to the problem.