Intermittently, the engine runs with an advance.

Intermittently, the engine runs with an advance-and-retreat motion, system is burning rich. I can smell the exhaust. At one point, I could not back out of a driveway. The engine would start, choke and gasp, then die. I had it towed to a mechanic and he ran the throttle by a lever under the hood and it cleared. What is this likely to be? I have had a tune up and an oil change last week which keeps the problem under control a little better. It still occurs.