These cars have a habit of a stumble.
First thing you can try is pull the negative battery cable for 15 minutes, that will reset the ECU. The car will take about 15 seconds of cranking before it starts, that's normal.
Or you can leave the key on the on position for that time before you start it and it will start normal, I have heard.
Get the car to a Tuner and have them street tune the car. Read in the Tuning Forum here and learn about the cables for your lap top or buy a Cobb AP v2.
Some of us put two washers between the Y pipe and muffler flanges to give it more sound.
You may also want to buy a can of MAF sensor cleaner and clean it before the tune.
174,000+ miles, 5/20/12 new ej257 short block, 8/20/11 installed VF52 turbo, @20.8psi, 280whp, 300ftlbs. Also CHECK YOUR OIL, these cars use it.