Cadillac transmissions

I have 2006 Cadillac STS with the Northstar V-8 with 52000 miles on it, and am experiencing transmission problems when the car accelerates from 40 to 60 miles an hour. I’ve had the dealer install a new torque converter, and reset the computer, but the problem stil exists. It seems like the trans is trying to shift gears and causes a vibration that only stops when I let off the accelerator. It seems to happen only under acceleration and when the car is going up an incline.

Any Help?