MY car has been doing this for a month, when I start the car it idles up to 1200 rpm and down to 400 rpm, it dose this for 15 seconds. It has been kicking on two codes they are p0106 witch is the map sensor, and the other is p0128 and that one is the coolant temp sensor. I have replaced both of them and the car still dose this. I have a k&n short ram intake on the car. I have had it on there for 2 years and had no trouble with it. One thing that i have noticed is that my care dose not make the whistling noise that is used to when you started it, my car has sai on it. It is a cobalt 2 door coupe LS 2.2L.

 

Possible Vacum Leak maybe