2005 Pontiac Grand Prix enigine jerk

Hey, I just read your question, I'm not sure if you even still have the problem or not. I own a 2005 grand prix and had the exactly the same issue. I fixed it with one thing. When you had the transmission flushed, did they use Wynn's additive? If they did not, you should really take it in again and get it flushed with that additive. I had it done about a week ago, and it is happening less and less. I was told it would take about a week to work. The problem is the valve box in the transmission, there are solenoids and valves that are building up with gunk, and causing them to stick. Wynn's additive breaks that specific gunk up. Hopefully this helps, I know it's a very problem that's pretty specific to grand prix.