The dealer and an independent transmission shop have not been able to explain a hard shift up when acclerating after a turn or slow down to under 30 MPH. the dealer thinks it is the torque converter disengageing and reengaging and no fis is needed. I am not convinced. Neither the dealer or the independent shop have found any trouble codes and have no recomendations.
How many miles is on the car and how many miles on the transmission fluid? You might need to change the fluid. I have used NAPA transmission conditioner. It is a conditioner for the o rings and seals, they can get stiff over time and this helps and is inexpensive to try. You might have to siphon out a little fluid before you add it so you don’t everfill the transmission.
Someone should also check the mounts.
How about telling us the year and miles on your Lesabre? Don’t forget to tell us about how old and the condition of the transmission fluid.