Engine Shut Off Without Warning problems of the 2004 Ford Mustang

In the morning, after driving my car approximately 10 minutes, (before entering the freeway on ramp) the engine just shut off. I was able to turn the key to the off position, and while still moving (in second or third gear) I was able to restart the car on the first attempt. I did use the vehicle mid-day without any mishaps. Later that evening after driving home approximately 40 minutes, (after exiting the freeway) the engine shut off again (while in third gear). I was able to again turn the key to the off position, and while in motion I was able to restart the car on the first attempt. On the drive home that evening and the next morning there were no further mishaps. I contacted the Ford dealer that evening, and was advised to bring in the car the next morning. I was not advised to have it towed or that it would be unsafe to operate. The mechanics drove the vehicle for 12 miles, and were unable to duplicate the motor turning off. They advised me that they had thoroughly checked the car, but were unable to find anything wrong, and the car was ready to be picked up. I was expressed my concern repeatedly about driving everyday on the freeway to their representative, but was offered no explanation as to why the engine just turned off. At this point I continue to be concerned about the safety of driving this vehicle.