2002 Dodge Neon

Engine light on. Been reset several times. Can't find reason for the light to be on. Other than a possible sensor which my mechanic assures me is no big deal

But.

left for work (car warmed up), drove approximately 10 miles. Stopped at stoplight (in drive), light green, gas pedal to go, and car jumps like a jerking motion, then goes, no spurting noises or stalling, just a serge/jumping motion forward. Pull over check hoses, tranny fluid, etc. Nothing looks odd. Drives fine the remainder 7 miles I have to go. I got in it at lunch to drive around the parking lot to see if it does it again. Putting into reverse, hard jerk, neutral, not as hard of a jerking motion, drive - seems find slipping into that. Proceed to drive and I hear this noise that sounds like something "flapping against the car" under the passenger front fender. I see nothing when looking into the hood or underneath the car. Hoses and belts all snug and in great condition. The radiator is brand new. Had to replace when the transmission tank broke and tranny fluid ended up into the radiator. All of this was flushed out and transmission filter changed. This was about 9 months ago. Only thing done recently has been the front brake pads changed.

should the tranny be flushed out again and refilled with new tranny fluid and a new filter again, along with the radiator being flushed and refilled? Will this solve this jerking forward motion.