I have been having the same problem. I currently have 47,000 miles on my G6. It has been doing it since about 28,000 mile mark. I have a warrenty on mine up until 75,000 miles. But I am getting no where with the dam dealer. Loughead Pontiac in Springfield Pa keeps telling me, Oh the G6 has the best transmission and we never had any problems. Blah Blah Blah, we plug it up to the scanner and can't find anything. They found 1 error code the one time, told me it was a bad shift sensor of some sort in the transmission, replaced it and set the shift points and I would be good. Lo and behold about 2 weeks later it was doing the same crap. Called up the dealer again, oh bring it in, not only was it shifting and making all this dam noises, i get a check engine light. I get it there, the light goes off, they plug it up again and say no error codes no nothing. Can't help you sir. What kind of crap is this. I can't help you, but oh let me let your warrenty expire, then we can do something for you. To this day, I have this problem and all they said, after I said I would get a lawyer because it has been 4 occurance, well you go do that and call GM and fight with them, since we cannot do anything unless we have an error code. What kind of mechanics are these. You don't need an error code to fix that, you try and pin point the cause. When the customer is telling you something, go and look for it.
Wow. I'm not sure if I have any trans issues (I have so many others, after a month and a half, I wouldn't doubt it) but I have had that EXACT same "well, the light isn't on now, we can't do anything" or "we can't be paying our guys to WASTE time diagnosing things they can't see" with my dealer in DE too.
I thought I'd start a case with Pontiac and did so, but even that has been a battle of waiting and still no one even getting a call after a week. BBB and other filing soon, perhaps.
Though...I originally popped in this thread after, truthfully, noticing some oddly harsh shifts at time lately driving. Doesn't seem to be an all the time thing, but as with all else, curious to see if it was common or not.