2006 Chevy Malibu RPM's bouncing

I have a 2006 chevy mailbu and recently I've noticed the RPM's bouncing when coasting. The "bounce" can be described as going from high to low then back again. For example only while coasting, the RPM's will be around 1500 (say that is the normal coasting RMP), the RPM's will drop to 1000 then back up to 1500, drop to 1000 and so on. It seems the engine is choking down on something. The problem goes away when accelerating. I have not noticed any performance issues other than this. The car accelerates well, does not hesitate on quick takeoffs and the car only has a slight roughness to idle. Just to double check and make sure the transmission wasn't causing any problems I can encounter the same coasting scenerio and shift to neutral and the RPM bounce problem still occurs. Any feedback or thoughts would be much appreciated