Do we have any electrical system guru’s on here?

[767Jockey]

ARRRRGHHHHHH - This thing is driving me NUTS!!

I installed a 560 ohm resistor in the line that goes from the alternator to the IGN terminal on the ignition switch. I start the car and it does the exact same darned thing. It gives me 13+ volts, and within a couple of minutes it drops off line down to no charge at all. Both the voltmeter in the car and checking at the battery with a VOM confirm this, it's about 11.9 - 12.3 volts after a while.

-I have checked all the wiring. It is correct.

-I have checked all the wiring connections. They're all soldered, heat shrink wrapped and solid.

-I have checked all the grounds - battery to frame, engine to frame. They're all clean metal to clean metal and tight.

-I have checked the alternator on three different auto parts store alternator testing machines. All test good.

What the heck am I missing here? Could it be the alternator is faulty? Maybe it works initially then drops off line as it heats up or something? Other than throwing another $140 alternator on it how could I test that? Any ideas out there what ths could be? I'm pulling my hair out in frustration now.