Mystery hesitation and low volts..

Dear all 328/308 experts. I have been chasing a mystery hesitation. I have been chasing a mystery hesitation on my '86 Mondial since I bought it in back in June. Symptoms are a strong hestation when warmed up, when under load. gradually gets worse until car stops running. Restarts fine after 1/2 hour. I have done quite a bit, changing fuel filter and pump, plugs, coils, coil transistors. Checked the ignition with an oscilloscope.

Just this week I've noticed the battery voltage drops from 14 volts to 12.3 volts when the cooling fans engage, then remains there even after the fans stop turning! Then the voltage gradually drops down to about 12.1 volts after 15 minutes watching it, so it may even go lower than that. Its like the alternator has stopped functioning and the battery is just draining. Battery recovers to 12.6 volts after sitting. I have checked the alternator belt tightness, and checked the B+ cable to the starter. I also checked the quick-release B+ cable connector by the firewall (I bolted the ends together as the plastic clip was broken and it was loose). Today took the alternator to a repair place. They bench tested it under a load and told me it output about 13.9 volts under an 80 amp load, so they dont think there is anything wrong with it. However, they did not heat the alternator while testing, so conditions not exactly the same as when I am driving it, getting it warm and getting the issue. Any ideas would be most welcome....