I played the "I think it is this" game with my last car. It to was an intermittent problem and after replacing relays, starter, ect I finally took it to the dealer to have it diagnosed. Turned out to be a short in a wire to the ECU. If you get no codes do yourself a favor and get it diagnosed by a professional. IMO you're risking your's and other's safety by driving your automobile in it's current state.