2 things come to mind:
#1- fuel pump relay: you will here a click when you turn the key. Listen for the fuel pump to turn on when you turn the key to the [accessory] position. If it doesn't turn on everytime, the fuel pump relay is probably bad. (est. $14)
#2- TFI module on the distributor. Many Ford vehicles have these(including your mustang) and when they get old they get burnt out. What happens is when the car gets up to 200° or so the car will die. You will not be able to restart it until it cools down. (est. $60)
Oh and if it is one of these, your dealer's mechanics are idiot's, shame on them!
I wonder if anyone could add to these? Hope I've helped.
