Trouble shooting an issue like that through a forum isn't going to be the easiest thing to do. There's just too many variables involved as to why the car is doing what it's doing.
It could be any of the things below.
Tune
fuel issue
ignition issue
voltage issue
bad FRPS
My best suggestion to you would be to take the car to a mustang specialist in your area and have them trouble shoot it for you.
If there's anything else we can help you with please let me know.