Change your fluid and filter. http://www.gmforum.com/t278423/
Make sure you get the correct GM transmission fluid. You will need about 7 quarts, a filter, and a pan gasket. The only special tool you will need is a inch pound torque wrench to torque the pan bolts to 120 inch pounds. I suggest the clicker style. I have used both a clicker and a dial type. The clicker is way better. Follow the instructions I have laid out in the write up and you will be fine. Takes about a hour and a half or so. I would guess the filter is getting plugged preventing the pressure from building up to shift.