Pontiac Repair: hitch under power, obd2 scanner, pontiac grand prix

Question
2004 Pontiac Grand Prix 3.8 automatic 89000 miles. Awhile back I replaced the plugs on my wife's car. Since then it has a hitch when it calls for more fuel,like when you step on it to pass a car or going up a hill. Even on cruise control. Once in a while it will not start on the first try. It don't skip or hitch on flat ground unless you step on it.
I have a OBD2 scanner but it don't send up any codes.       I would like to thank you in advance for any help you could give me in where to start looking. Parts are expensive to just guess and buy.

Answer
Hi
There are WAY to many things it could be, as you state, even for me to venture a guess.
Change the fuel filter. That never hurts.
Plug wires also. If it is just a slight miss, It MAY be a coil. Does your scanner allow you to observe data?
If so, watch the misfire data and catch a snapshot when hitching. You MAY see which cylinder it is.
With this info, you can pinpoint the cause.
I hope this helps.
Sorry for the delay.
Good Luck
Pawl