I still think it's a ground strap, maybe just a loose/rusty connection. Make sure that you have braided straps from the engine to the firewall, from the firewall to the (-) battery connection and from the starter to the (-) battery post (that one is a black 6-guage battery cable). Make sure the stud on the firewall is welded tight and does not turn at all, and that all ground connections are clean and tight.
If there is any doubt about this, run a seperate 6g black wire from the alternator case directly to the battery (-) post. Your problems appear to be alternator related, so it's suspect.
Years ago I replaced the stock 6g black and red battery wiring with 4g wires. But that's not required, I just did it as part of a dual battery upgrade.
1967 Kaiser Commando
2001 Grand Cherokee WJ
2003 Wrangler Rubicon TJ