Engine smooth idle but would not accelerate?

This problem started a few months ago when my wife parked the car in a plaza parking lot for a couple of hours. When she returned, the engine started fine with a smooth idle but would not accelerate beyond normal idle rpm. Our local garage had to tow the car back to their shop. They had the car for several days but were unable to figure out the problem which was intermittent. Sometimes it would start and run with normal engine acceleration, and other times it would start but not accelerate at all. I got the car back home and the problem worsened. More often than not it would not accelerate beyond idle. On February 4th I had it towed to a trusted Chevrolet dealership. As of today, February 12th, they have been unable to find the problem due to the age of the vehicle and the unavailability of engine information from GM. Apparently, cars have changed enough since 2002 that some problems (like mine) are difficult to figure out. The service manager has kept me informed and shared the many different systems they have checked so far but without success. I’m hoping that you have some ideas that might help. My email address in Florida is nickprisco@hotmail. Com