I have the exact same problem on one of my cars. I tracked the problem to the vacuum valve in the ignition switch. The way to tell is next time it happens, disconnect the vacuum line going in to the top of the IP, as stated before. The stop button won't be depressed. If this is your problem, the line will be hard to disconnect, because it's under vacuum, you'll hear the vacuum escape, and then your car will be able to start.