First thing I would suspect is hydrolock. Common problem with bikes that have sat a lot. Gummed up carbs will let a small amount of fuel through into a cylinder and stop the motor dead. This can break a bunch of stuff in the motor, and is a pain to fix. The next time that happens, Don't keep hitting the start button. Pull the plugs, (all of them), then give the motor a spin. If this is the case, fuel will shoot out of the plug holes. (Make sure your are clear of the garage and stuff). If that is not it, next would be the battery. (In my mind) You can look up hydrolock on the tech board by going to the top of the tech page to the search box and enter Hydrolock, and read thousands of pages on the issue