I just had a similar problem with my 2001 GT 3.8L. My car would buck even at low speeds. The bucking would come and go while driving. I cleaned the MAF sensor and the problem temporarily went away. After a week the problem was back. I read an article that the MAF sensor design was prone to intermittent problems and would cause this type of fault without a service engine light. I replaced the sensor and after several months of driving the vehicle can safely state that the problem is gone.
Hope this will help.
Pat53