Why does my galant jerk?

I have a 2000 mitsubishi galant v6 and recently i noticed that if the car is cold, there's a SLIGHT hesitation from Park to Reverse, but once its been driven for a while, the jerk is more intense. Also, if i roll the window down, i can HEAR the gears shifting from like Park to Drive, etc. Its sorta a click sound. Took it to 2 tranny shops who said it wasnt the transmission. NEED HELP!!