'66 Dart won't start

'66 Dart 273 with Pertronix 1 ignition kit with std distributor. Stopped to get gas and car would not start. Was running just fine. Getting gas, turns over but will not fire. When letting go of key will act like it wants to fire momentarily every time. Changed ballast resistor (2 times). Checked connections and looked for loose or broken wires. Any suggestions on where to go next? Could this be an ignition switch. Replaced it a few years ago purchased from Van's Auto. Thanks