03 GTP with 110k miles, tranny service performed about 15k miles or 18 months ago. After a 50mile highway cruise, I came to a stoplight and when leaving in 1st the car moved, hesitated and then bang, off it goes. Trans temp was about 175F. The highest temp I've seen is 188F. I checked the fluid and the level is right and it looks absolutely clean as new. Stock PCM.

This is the second occasion it has happened. So what am I in for?