I noticed this on my 2007 Pontiac G5 during a bumper-to-bumper traffic jam that my 1-2 shifts (and to a lesser extent, my 2-1 shifts) would get progressively harder. The shift roughness kind of follows the coolant temperature, but the problem doesn't really go away when traffic starts moving and the temperature drops to normal. You can see and hear in the video just how bad it is.

[yt]https://www.youtube.com/watch?v=QfHu_592Iuw[/yt]

When this happens, the 2-3 shift is a little rougher, and the 3-4 shift is maybe a hair rougher. Oddly enough, if I rev the engine higher, the shift is smoother.

I was able to replicate the problem taking a long drive on back roads with rolling hills. The 1-2 was nowhere near as harsh as in the video, but my passenger started asking questions.

The only real fix is to park the car and turn the engine off, which makes me think that my transmission either needs a rest; or the PCM is commanding higher/maximum pressure and not bringing it back down. No stored or pending codes.

I have a powertrain warranty, so I'm not too worried about the fix. I'd just like to have a better idea of what this might be, if it'* a well-known problem.