My car surges at idle and low RPM

My 2002 Pontiac fire bird surges or stutters at idle and low rpm I have only owned the car for a month and it ran like a top u til the first time I washed it then it started to surge a couple days later it cleared up no problems ran fine for a week washed it again same problem no service light or any thing until this last wash my service engine light came on
I decided to reset my ecu by disconnecting the battery drove the car 40 miles with. No problems all around d town today work ect no problems ran fine all the way to the gas station as soon as I came out of the gas station. And started it the car started to surge or stutter.. What is going on I have no clue what to do if u get on it it pulls out of its stutter, surge runs but. As soon as it shifts and hits lowrpm it starts all over again