Look into the electronic theft control. My Excursion would do that every so often until I got stuck in NY for 3 days while they figured out there was a wire broken in the steering column that went to the key reader that's around the ignition lock. Look for the theft light to stay on while you're cranking it when it won't start. It's a pretty common problem on Fords. They save money by shortening up the wire leads around parts that move until they stretch far enough to fray and then break. I've added wire to leads for the electric windows because the wire breaks where it comes through the rubber weather hose between the door and the body from opening and closing the door.