Keep checking that cap and rotor. Could it possibly be an advance weight at idle that swings free when you rev it? It may have lost a shaft bushing allowing it to move up and down under load. Pull it and see if there is more than about .010 axial shaft play. Should be almost nill.
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rotor should swing in under contacts as well as past (step shaped) them so too much axial play would tear up the contacts on top of rotor. See the step? Makr sure rotor is seated. Need them pics!~
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