Battery side or starter side was to your question. You have found the problem with the 6.6V on the blue wire. This is also most likely the reason you had no spark. I would first check the wiring for any damaged/frayed/burnt wiring and also make sure all grounds are tight. If there doesn't appear to be any problems there then check the ignition switch and safety switches. You can search youtube videos for testing these(easier to understand) and maybe someone can provide a wiring diagram to help. I hate intermittent problems because they can be difficult to diagnose.