I had a similar issue, although with a much different trans (GM 4T65E). In my case, it was the fluid pressure control solenoid sticking. Replacing the solenoid cured the problem permanently.
I would recommend getting the car to the point where you can easily replicate the problem, then stop by the dealer to have their trans guy take a ride with you. You'll probably want to call ahead to arrange this. In my case, the dealer didn't want to make a solid diagnosis but based on a TSB, I went ahead and did the work myself. It took all day, but was worth it. The dealer wanted to just sell me a rebuilt trans.
I would recommend getting the car to the point where you can easily replicate the problem, then stop by the dealer to have their trans guy take a ride with you. You'll probably want to call ahead to arrange this. In my case, the dealer didn't want to make a solid diagnosis but based on a TSB, I went ahead and did the work myself. It took all day, but was worth it. The dealer wanted to just sell me a rebuilt trans.