Bad battery ? From the diagram, if it is not the alternator, battery or regulator, then there is an intermittent short somewhere. There is an electrolytic capacitor hanging from the main distribution system, if this is shorted in a voltage dependant manner. But it would explode if it were drawing so much current. The only part that I know of that could draw so much current without exploding or making sparks is the battery.

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Saumil S. Patel