1992 Ford Bronco High RPM

I have a 1992 Bronco with a 5.8L engine. The vehicle runs fine for the first 30 to 60 minutes. Then the engine will intermittently increase approximatley 500 RPM while driving and the check engine light will come on, then return to normal. The vehicle speed does not slow or increase. The problem also shows when stopped. When I let off of the brake it will accelerate on its own. I have taken it to a local repair shop, they say it is not storing any codes. They had the tester connected while the check engine light was on and it showed nothing.