Again, same post I made on the Malibu forum. COMMON issue, at least so far on earlier (2007 mainly) Auras with the same engine, but also oddly the 3.5L as well:
Actually, sounds similar to some issues there were with early Saturn Aura's with the 3.6L (identical in every way under the skin). Owners reported random shaking at idle as well as that feeling under deceleration. Oddly, same thing with the pushrod 3.5L used in a lot of Aura's and G6's still, and I'm having a bit of it in my G6.
I believe it's a programming issue related to the lean/rich fuel/air mixture and something else. But what's odd is I haven't seen this brought up in relation to any '08s, Aura or Malibu, but you never know.
Here's a couple of the Aura threads discussing similar things:
http://www.saturnfans.com/forums/sho...ght=rough+idle
http://www.saturnfans.com/forums/sho...ght=rough+idle
http://www.saturnfans.com/forums/sho...ght=rough+idle
http://www.saturnfans.com/forums/sho...ght=rough+idle
Especially in the first link, the symptoms sound IDENTICAL. Still weird that an '08 Malibu would suffer from any of it, but it can happen. Feels like it's stumbling or chugging? If so, I think that's because it is. Same thing with my '08 G6 3.5L, despite being a different powertrain.
There may be a computer update but as I haven't heard anything much about this, it's only a guess. Hope this info helps!