Well first of all, you say rarely have it out. So battery may be the issue. Ammeter needle should show a charge for a little while then settle in center area. So two tests need to give a direction to go next.
1. Measure voltage across battery posts before starting.
2. Measure voltage across battery posts at high idle.
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Could just be corroded cable clamps, posts, cables, other connections. Neg and Pos.
Or of course, bad battery, bad charging components. Bad grounds.
Need the two tests first,
