Need to get 81 VW Van running. New battery, coil, fuel filter & fuel pump. Pulled alternator out and had Auto Zone test - said “within specs.” When battery full charge (after on charger) the van starts and first run around the block is fine, even with throttle down for power. Then idle for a while and try again, doesn’t run fine. Pushing throttle down for power, or to rev up, makes engine bog down and stop. After this, if I turn off the engine it won’t start - must charge battery. Two clues. (1) Voltage at battery with engine running is never above 14 volts. (2) If I turn on headlights and emergency flasher when idling the engine slows down and wants to die (voltage at battery drops at least half a volt. I still suspect alternator or battery, though both test good.
As long as the volt meter shows 12 volts or above, the battery and alternator are probably fine. I’d suspect something in the ignition system is breaking down with heat.
Sounds like a pretty serious short is a possibility. Inspect your positive cables for issues.