I had precisely this problem and it was sorted by new distributor cap and arm.
It really showed off. Even with a brand new battery it would show 9v all of a sudden then behave almost like it was totally flat.
Come back to it, right as rain and totally unable to replicate the fault. Frustrating but rotor arm and distributor cap solved itm.
The cap is a little bit tight to get at as it is close to the bulkhead.
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