1989 Plymouth Voyager Car hesitating + makes pop sound/Fail

My minivan a few months ago started making a popping sound from under the front of the vehicle. It also hesitates when going up a hill, at stoplights/signs, and in some cases stops on me. It also failed emissions because of elevated HC levels. I had a tuneup (new sparkplugs/wires, dist. Cap, rotor button etc.), Oil change, new alternator/battery but still having same problems if not worse. Now when I even try to start the engine when it trys to turn over makes pop sound and once I put it in gear it just shuts of on me. I also have a check engine light that comes on and off. What should I do?