help

I have a 2003 Mustang GT 4.6L V8 automatic. I am having some problems with the running of the car. When I am crusing around 40 to 50 mph I get some hesitation out of the engine. Sort fills like it is missing or hesitation. It is very furstrating because I have taken it back to the dealership and they said it is running fine. I dissagree. Can anyone give me some idea of were I need to start looking. Thanks

dealerships are worthless. take it to a reputable independant shop.