It won't be your plugs or plug wires so quit looking there.
Battery is a posibility, but more likely the starter or battery/starter cables.
You need a voltmeter to better pin it down. When it won't start, put a voltmeter across the battery terminals and confirm that you have about 12.5 volts. If not, have a good look at the battery. If you do have good voltage, check to see if you have good power to the starter. Don't forget to check the battery ground cable, both ends.
