You are not losing your mind...which is what my dealer has tried to convince me. This board is a God-send! Another G6 owner told me about it. He had happened to stop me randomly in a parking lot to ask how I liked mine as he had bought his wife one...turns out, we had many of the same issues...and there have been MANY! The vibration you mention is one of them. I too have been having that problem since approx 3 months after purchasing my 2005 G6. 90% of the time, it happens on the highway - when slowly accelerating - when the car hits around 55. However, I have also had it happen around 40 miles an hour on in town roads and at higher speeds on the highway. It actually STOPS accelerating at that point and I have to push harder and harder on the gas....through the vibration....it will begin to accelerate slowly (no where near as much as it should for the pressure being applied to the gas) and then at about 62 or 63, it's almost like it wakes up and the gas kicks in, and off I go! FIVE trips to the dealer...told them from day one about the associated acceleration loss and that it seemed like a transmission problem....almost akin to driving a stick that has a clutch on the way out and the gears start slipping. They balanced my tires....THREE times! I told them that wasn't it...apparently, being female, they just assumed I didn't know what an unbalanced tire "wobble" feels like...but they did manage to make a whole NEW and different "vibration" in doing so - a TRUE off-balance wobble! LOL On visit 4, they made me take a tech out in the car (which I had asked for from visit 1). His answer..."I think it's just the gears changing".....AH, NO...I DON'T THINK SO! They then set me up for an appointment at a transmission shop...they kept my car for three days (while the dealer put me in some piece of junk 1990 something Grand Am with 120k miles on it with bad breaks and one headlight....but that's another story!) It was returned to me and they said they could find nothing wrong and that they "couldn't duplicate the problem" They must be out of their mind because I don't even TRY to make it happen, it just does....every time on the highway, regardless of whether I drive or my husband drives...so I have no idea how they could not "duplicate" it. The bad news is...I have no solution for you. I finally gave up and figured, whatever it actually is, something is going to eventually break completely and at that point, I have all my records showing I brought it in and I will just say "told you so". But, I hope that it does help in some small way to know that it is not just you having the problem...just in case your dealer, like mine, tried to convince you of that! Good luck! If I ever come across a solution, I will be back online posting it!