Ever have a replacement or duplicate transponder key for Ford, Mercury, or Lincoln made and it would not program to your vehicles computer? Your old keys worked just fine, but now the new keys won't program to your vehicle so it can start. Try checking the PCM fuse relay in your vehicles fuse panel. Many customers report the car hesitating to start or the immobilzer (anti-theft) light coming on intermittently while driving or attempting to start their vehicle. This is a sure sign the PCM fuse relay is busted and needs to be replaced.
Also if someone attempted to steal the vehicle, it is possible the transmitter ring located around the edge of the ignition lock under the plastic shroud is cracked or broken. The transmitter ring is made from plastic, has an electronic connector whose wires are connected under the shroud, and are very delicate.
Attempting to do your own ignition lock replacement can also cause this problem. Leave such work to an automotive locksmith professional like our company.