Just to be sure about the negative battery cable, you can make a cable about 2 feet long with 8-12 gauge copper wire with a terminal attached securely to each end.
Using a 10 mm nut, connect one end to the unused bolt on the negative terminal and to any place convenient on the engine.
If your problems go away, you have a ground or internal cable issue with the negative battery cable.
Fix it, and remove the temporary ground cable.
( I am assuming you have already cleaned the battery posts and cable clamps )
You could also have an intermittent open internal to the battery.