Mine had this problem intermittantly and it started happening more often than not New ignition switch sorted it
Also make sure the connector going to the ignition switch is on properly, you should see a white connector under the ignition switch, give it a push up or wiggle next time it dont start and see if it fixes it, mine is quite loose. You could diagnose if it is the ignition switch by pulling the connector off the ignition switch and hotwiring the terminals, though I cant tell you which ones to link of the top of my head. Alternatively you could connect a new ignition switch to the connector after you have pulled the connector off and turn it with a screwdriver, that way you can confirm whether its the switch or not before you go to the trouble of taking the old one off, its a bit fiddly to do.
