My truck is doing the same thing!! I am begging to think that maybe it is because of the colder weather???idk but the one thing i like about the winter is when my truck idles up and the engine warmer thing when that exhaust shutting valve starts closing and with the straight pipe it sounds like jet engines speeding up its sweet!!!!! i get alot of people thinking my truck is breaking or something and i just tell them its my jet!!! lol but i get some people thinkin its perddy cool... I hope my truck isnt screwed up with the serging at regular idling serging