2000 E320. Yesterday tried to start (supermarket carpark), starter motor cranked engine as normal, motor didn't start, just kept cranking. Switched off for minute or so, tried again started fine. Same thing happened later in the day. Doesn't sound like battery but tested it anyway using posts under hood. All off: 12.5 volts, ignition on 12.2v, lights on 11.95v. Sounds electrical/spark problem. Any suggestions?