Use a digital multimeter across battery posts, everything off. What is the reading? 12.6 volts is generic spec for a full charge. If battery is down, get it charged. Make sure battery connections are all clean and tight. Check charging system. Did you check for voltage at the starter, When problem is ongoing? Any testing, make sure tranny is in park or neutral and the parking brake is set. The starter solenoid s terminaL goes hot with key in the crank position. The other is hot all the time.
When the problem is ongoing, did you try cranking in neutral?