Need to check if you have any spark, if not it could be a bad coil, but I would almost guarantee its a bad ignition switch or connection, or possibly neutral safety switch, try to start it in neutral rather then park when its acting up.
When it isn't working check the positive side of the coil for voltage when the key is turned on, no voltage, is a bad switch or wire.
Or it could be simply bad points if it isn't driven very often.
Good luck!
