I just dropped off my car at the dealer for the second time (still under warranty)

The History:
(2008 Limited Coupe with 26k on it) I bought the car almost new (250miles on it) and it always had a idle problem it seems like it idles too low and the needle with bob up and down when I am stopped and in gear.. If I rev the engine (put my foot on the brake and step on the gas it smooths out) the car is not throwing codes on the dash lights.

The Problem:
Last Friday I had a small fire which fizzled out by the airbag light in the center console.. The car seems fine everything works mechanically and electrically. But I wanted to bring it in just in case. I told the service writer about the fire and I also told him to have them check the idle. After a couple of days.. The tech went through the car and said he could not find any burned wires or any kind of burned anything.. and that the car is performing normal.

I picked up the car at night right before they closed, at first the car was fine as soon as I got to the first stop light and tried to accelerate (in a normal fashion) As I pressed on the peddle at first the car did not respond then it started to accelerate.. (that's the new problem)... The next time it happened was yesterday when I was driving down the road and the light was red as I was approaching the intersection the light turned green so I pressed back on the accelerator which did not respond at first but, then kicked in.. once I got to the next red light the engine stated to shake like it was about to stall out.. I let my foot off the brake and let it roll forward and it caught up to itself. No lights lite up on the dash.. and lastly I got to the next light where I make a left. I was at a complete stop the car seemed fine and when I went to turn (left) and accelerator problem happened again.

Today I brought the car back to the dealer and took the mechanic for a test drive. I could not duplicate any of the problem. ("they can't fix it if they can't duplicate it").. not sure what to do.. any ideas on what it could be? the car is still at the shop I told them to go through it one more time.

Thank you in advance for your help!