Well that's not good - that symptom wouldn't present if it was just a stuck starter relay.

The tach signal would almost certainly be coming from the CDI control unit and it appears to be confused.

Are there any codes in the N3/9 CDI ecu?

As a pure guess could a bad crank position sensor be feeding a spurious signal into the process that's making the ECU think the engine is actually running when it's not? That would explain the no-start when the tach is moving around prior to starting.

I generally don't agree with the crowd around here that proclaims The-240D0-OM61X-is-the-best-thing-ever-made crowd but in this situation they are absolutely right. Even with a SDS it's very difficult to diagnose this kind of intermittent problem and once you get into the mysteries of the black box ECU there are really no possibiliites of fixiing anything once you verify the inputs are correct.

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