2000 Saturn SL2 constant acceleration/check engine light/po

Hi,
My 2000 Saturn's engine light has been going on and off for about 2-3 years now, but the car ran fine. It would go on for a month, off for 2 weeks and back on. My mechanic tested it for a "vapor" leak, but couldn't find one. He also cleaned something or other just in case. He recommended tightening the gas cap well after filling it, but that didn't solve the problem. We were not concerned because the car was running fine. After about 2 years of this, the car suddenly began acting as if the gas pedal was being pushed, even when it was idle. It is now in a constant "acceleration state." I guess I should mention that my daughter began driving the car about 6 months before this acceleration problem, and I'm pretty sure she doesn't drive it like I do : O ! Thanks so much for your input.