2001 Chevrolet Prizm Speed Control?

My car runs fine as long as I don't go over 45 mph. When I reach 45 mph the car seems to slow, even with the gas pedal to the floor. Most of the time I have to regulate speed by tapping the gas pedal and releasing the gas to gain speed. The car loses power when I climb hills even though the gas pedal is to the floor. I was wondering if it could be something as simple as a fuel pump, or if it was something that has to do with the engine? My mom got a diagnostic done on it over a year ago, and it said it was engine control. I have talked to various mechanics, and all of them had no idea what that was, nor how to fix it. I would like to know if there was another way I could find out what is wrong with it, besides taking it into a car repair shop, which I may have to eventually end up doing.