engine surge

Under light load/acceleration, the engine with have surge between 1500-1600 rpm. It is worse and more pronounced if the car isn't driven for a while. Seems to lessen with frequency after the car is driven 1/2 hr or more. This is sometimes, not always accompanied with a harsh shift from 1st to 2nd and 2nd to 3rd. Dealer has replaced the torque converter, but problem exists. Does not throw any trouble codes so dealer is at a loss of what the surging issue is.
Any thoughts or suggestions will be most appreciated.
BobC


