Intermitent problems are the worse kind to diagnose and I hate working on customers cars that have them. Obviously you want to check all connections but I assume you have or had someone else check them for you. Also have the starter checked.  If it continues, I would suspect one of two things. Ignition switch or the ECM. Either one will do it and you will need a good mechanic to check them out. If you want to take a chance and start throwing parts at it, I would replace the Ignition switch first as it is the cheapest of the two and you wont need to take out a loan for it. Replacing the ECM can get costly. If you decide to replace this yourself, make sure you do it correctly so you dont damage the new one.   By this I mean bolt it in place before you connect the plugins to assure a good ground to it. Always disconnect the battery before working on these parts. Good luck.