All the cables are new. I replaced them when I replaced the starter/battery/alt. I cleaned all the grounds before I put them back on.

 

The alternator was the only thing that was unreliable on the 89 Formula 350 I used to have. I learned to keep an eagle eye on the voltmeter. If the voltage was at or over 13v, alternator was kosher. If it started to drop under 13, I knew I had to make a run for home, cause another one was getting ready to die. One time I didn't notice the voltage had dropped. The radio dying was my warning, I barely made it into my driveway.

I bought an AutoZone alternator with a lifetime warranty the 1st time it died. Got 3 more free replacements over the next few years I had the car. So just cause you have another alternator in there doesn't mean it's a good alternator.

Another possibility is that the excitor wire is flaky. This wire feeds 12v to the alternator to get it charging. If it doesn't get the excitor voltage, it doesn't put out currrent. I'm not sure which terminal is the excitor on that model alternator.

 

Absolutely correct. As an aircraft avionics tech for 32 years I've seen this kind of thing a lot. Our cars are ageing and the grounds and connectors are too. I make it a habit of cleaning grounds and connectors when possible. Have you ever measured the voltage at each headlight? I did about .5vdc difference lower from left to right. I still need to correct that. That is why our headlights are dimmer than they should be.