2004 Chevy Impala Engine stumble

For the past week every cold 25 degree morning I have driven this rarely driven Impala SS its my second car and when the vehicle warm up I put it in reverse to back out the garage and the engine idle drops as if it wants to cut off but then the idle revs up and the car runs fine no check engine light or anything has come on and it only does it when the motor is cold today I cranked it up and left it in park as it warmed up when it was warm I gave it a little gas and it did the same thing but immediatley goes away.