After getting a transmission service on my GP, I've noticed after the car is warmed up that the shifting is a bit rough at accelaration. It's new Dexron III in there now; should a switch to Dexron VI clear up most of the problems?
Weird that several other Impala owners have mentioned improved shifting by switching to the Dexron 6. However, may I assume you were shifting fine before and now you are not? Dexron 3 shouldn't create any issues, your car came with that from the factory.