I just bought my 2008 G6 3.5L today (30k miles on it almost exactly) and immediately noticed off the lot that at idle and between the 35 - 45 mph window that the engine kind of starts fluttering as if it is gasping for air. At least as someone who is not very mechanically inclined that is how I would describe the sound. During this flutter, the car shakes and rumbles a bit and you can see the tachometer fluttering as well while this is going on.
I have an appointment to go and get this checked out on Monday with the dealer but I wanted a few opinions first. I expect I will probably get told there is nothing they can do.
I have cleaned the throttle body (I've seen instances where the same symptoms were produced by a gunked up perimeter around the butterfly valve) but that didn't help at all. It does not happen in times of intense acceleration or any time after it has reached the 50 mph mark. A quick search reveals this is a common problem but I have yet to hear if anyone was ever able to get it resolved.