[RickMN]

My money is on a bad ground wire. Have you tried tapping anywhere besides the alternator? If you do and it starts, that will pretty much confirm a bad ground. Or try this. Keep a jumper wire with aligator clips in your car. When it doesn't start, try connecting one end of the wire to the firewall, fender, or radiator support and battery negative. If that works, you have a bad ground to body. Or use a jumper cable from the engine block to the negative battery terminal. If that starts it, you've got a bad battery to engine connection.