I had the same sputtering problem awhile back. I also noticed my car was idling harder than other cars I had owned previously. It would jerk when the engine would kick in. Then I began having starting problems whenever I would fill the car up or if it sat in the short amounts of time as you described. Finally, the car was filling with this aweful smell of fuel! This was about the third time I had brought it back (only this time with increasing problems aside from the sputtering) and it turned out I needed to have some major fuel parts replaced. Unfortunately, it hasn't been quite a year and I am have problems starting my car after filling up the gas tank once again.
I have also had the shifter knob replaced twice, a new ignition barrel and key, windshield wiper replacements (not the blade, the wipers actually stopped working altogether during one of our worst winter storms while driving), the module for my onstar was bad and they had to reorder it, the sway bar bushings have been replaced, and now they are telling me there could be something wrong with some celenoid?? Typical, it is going back in again for some weird noise that it has been seen for three times already. I am seriously considering getting a lemon law lawyer because this has been absoultely rediculous!