Sounds to me like it's probably somewhere in the fuel system. I would guess your fuel pump is starting to fail. Next time it wont start try this:

1.) After finding out it isn't starting, turn the key off and open the gas cap.
2.) Have a friend turn the key to the "on" position but not start the car.
3.) Listen by the fuel cap for an electronic humm or buzz. If you hear the humm/buzz than your fuel pump is working. If you do not than your fuel pump needs to be replaced.

If you discover that your fuel pump is fine than you may want to get your battery and ignition coils tested.

Hope this helps!