1st, you would think you need to go to whoever you bought the car and get the tuner for it...Xcal2, Xcal3, whatever it may be. Im not 100%, but Im pretty sure you cant go back to stock without the stock file that was saved on the original tuner equipment....not 100%.
Maybe its something in the tune...or lack of one, maybe its mechanical...why would you buy it if it didn't run right? did you test drive it??