I just read through this quickly, so I apologize if this has already been mentioned, but another good thing to do is to separate the Arduino power supply from the supply used for ignition. That way if the igniters cause the voltage to drop below the acceptable threshold for the 5v regulator on the Arduino, your control box wont reboot on you. It might also be a good idea to have the igniters physically disconnected from the control box while the Arduino is booting, so floating pins wont cause a misfire.