More than likely it's a pressure control solenoid that has gone out. My 2008 was doing that & I took the car to the local dealer that used to sell Pontiacs & they checked the car out & told me when that sensor goes out the torque convertor clutch might have problems engaging due to the bad solenoid. Best bet is to take it to the dealer to have them check it & replace the part. Not sure of the cost though, as mine was still under the 5 yr/100k powertrain warranty.