2006 Pathfinder hesitates at stops

[thetotalpackage52]

2006 Pathfinder, with 103K miles. Tune up performed at 100K miles. The problem I'm abut to describe occurred both before and after the tune-up.

The problem is that sometimes when I come to a brief stop and then step on the accelerator, the vehicle hesitates and then jerks forward. The problem also may occur when I'm traveling at low speeds, step on the brake, and then on the gas. The problem never seems to happen when I'm stopped for a considerable amount of time, like when stopped at a red light. There are no other symptoms. At high speeds, there are no similar problems. I replaced the radiator a while back, on the advice of my mechanic, to avoid the dreaded coolant/transmission mixing problem. TIA.