i have an 05 LS that sometimes takes a few to shift to drive from Park or Reverse. lately its been doing this when going from a coast (out of accelerator) to accelerate. not hard acceleration but just to keep highway speed. its not downshifting. the tranny fluid and filter is 20k old and looks fine. my thoughts are torque converter. it has 88k miles.

any thoughts? this isnt consistant but its happening enough for me to question.

 

My 06, 3.5 Impala was doing the same thing...sort of a delayed thump and then severe downshift to low instead of 2nd as you press down on the accelerator pedal. Thought the tranny was going to crater. Nothing to prove this now, but suspect the concept of shift by wire with an electronic transmission (4T65E) in a newer car. So now I don't push down on the pedal so quickly (like I could with a detent cable system) and she seems to do OK. This probably doens't help you much but maybe one of the tech's has had experience with this particular problem. Time will tell.