I am having issues with the way my 1994 Pontiac Trans Am is running.?

about a year and a half ago my car started stuttering sometimes when i pressed on the throttle. It didn't happen often so i never took care of the problem. About a month ago it started getting worse. Now it revs itself when it is in park. Also, when i am stopped at a light just putting my foot on the break it will try to take off. Also, when i give it gas sometimes a popping noise comes from the exhaust, kinda sounding like a backfire. I took it to my usual mechanic, he tried replacing a sensor but it didn't work and since the engine light never comes on it is difficult to tell if it is elctrical or something else. My mechanic now thinks it is the distributor cap but i had that replaced two years ago. Any ideas would be helpful. Thanks.