Car has been operating perfectly. But during a downshift up a hill with the cruise on, it shifted back to high gear with a definite lurch. The next several stop and go shifts were unusually abrupt. A while later, same day, another short trip revealed pretty smooth.

I have seen reported cases of this, along with Check Engine light coming on, My Check Engine came on a couple days ago. Perhaps they are related.

I have seen a case or two online where a transmission wiring harness change fixed this condition.

Update from Oct 2, 2014: Since posting the above I have done a lot of research. I felt the hard shift was related to Engine Light, which told me it was camshaft related. The dealer posed several options including up a new camshaft. I read things on this site and also the G6 Owner's club.
i concluded it was most cost effective to try this GM Part number 12628347 but new PN is 12655420 for intake for about $52. I changed it myself b watching a You tube concerning the 2.4 motor but in a Malibu. (The Pontiac was actually easier).

This immediately solved the hard shift problem and is fine after 500 miles of mixed driving.