Sputters and hesitates

Thank you so much. I installed a set of spark plugs and the car has never ran better! I love this site.

I have the vehicle listed above 3.5L v6. What is it and can I fix it? I have already been to the dealership and they want to replace the computer and they want over $900.00 to fix it. I have read online that others have already done this and it did not fix the problem.

It has been stumbling while in gear and idling (at stop lights, in parking lots). This generally happens after a length of driving at consistent speed and then coming to a stop fifteen to twenty minutes of driving over forty mph and then stopping at a light is usually when it occurs, but it is random and not every time. The check engine light came on for a few days and went off. I had the code checked and it said multiple misfires detected. I took in to the dealer and they did a software update for the cylinders. A few days later the problem has resurfaced. If I shift into neural when it starts to stumble, it stops stumbling and regains a smooth idle. Also accompanying this issue while driving at low speeds and accelerating, on multiple occasions, the rpm's increase, but the speed does not increase appropriately and then suddenly, as if something breaks free, it accelerates rapidly and then drives normal at a steady speed. I am taking it back to the dealer next week, but I would like to give them some leads to follow so I am not going through trail and error.