2007 Saturn Vue pulsing jerking motion

My 2007 Saturn Vue had a tune up 3 months ago, after driving in heavy traffic approximately 30 minutes the car starts a jerking or pulsing motion, the car hasn't stalled out yet but you can feel the pulsing possibility of the car stalling. The mechanic feels it could be the reference sensor, we're just waiting for the part, or the transmission. The timing chain has not been changed. The car has 107000 miles on it, and has always had it's regular oil changes and basic up keep. I have no problem with the gears which I feel the mechanic is wrong thinking it could be the transmission. I'm leaning toward the EGR valve. But could it be something as simple as the fuel injection system being dirty from cheap gas?