2003 Ford F-150 big problem

I have a f-150 harly wth the supercharged 5.4l in it. The problem is I was going down the road and the engine just shut of and the theft light was blinking, and a little light that looks like a gas cap was on and the motor would not restart. The theft light was blinking very fast and I restarted the truck and it slowed down and was still blinking will it was running going down the road till it shut off again. I had the truck towed back to the dealer and when it arrived it started right up. The dealer said they could not find anything wrong. Well since it has been there the truck sometimes trys to stall out on me going down the road. When the truck is busy acting up and a short period of time after the left blinker indicater in the truck will blink fast like I have a bulb out. But when you drive a little bit more and turn the signal back on it is flashing fine. Also sometimes the theft light will blink when it is acting up. If your have any other questions feel free to call me at 443-807-2076, or email me thank for your time