Check the grounds from the battery to the body, and body to the engine, and battery to the engine. All three need to be good.

Once you know those grounds are good, remove the stud and replace. You could have some corrosion down in the bolt hole.

While that stud is out, start checking the wiring harness right there on the ring terminals. It's not uncommon on old harnesses to have wires broken inside the jacket, that appear normal.