Aaargh! Electronics for the electrically challenged?

[Y-TRY]

My car was doing okay and then my charging system just stopped. It's driving me crazy. I've gone through all the wiring and it's all okay. Everything is wired according to the diagrams and all the fusable links seem okay in the harness, etc. Even any questionable connections were re-done if I didn't like them. The battery and alternator checked out okay at 3 different shops. I've scuffed and cleaned every ground surface and every ground strap and wire. Still nothing.

I can have a fully charged battery and put the multimeter to it. It'll read good (about 12.70) at rest. I can start the car and it drops to about 12.0 and slowly goes down as the car sits there running. As long as the fan doesn't turn on I can unhook the positive post and the car will continue to run. The idle quality diminishes greatly but it'll still run (until fuel in the bowls runs out -electric pump). But if any other accessory is turned on, or the fan kicks on, it's simply more than the alternator can handle and the car dies (still with the positive lead off). It's like the alternator is running but not enough. The idle changes because the MSD and coil aren't getting enough juice, thus no zap in the plugs. So now I've got fouled plugs to boot.

I've got a 140 amp alternator wired in with a Painless High-Amp alternator harness. The big Maxi-Fuse is still good. It's internally regulated and the old regulator harness is bypassed per Painless instructions. Because it had previously tested okay and was running fine for hundreds of miles, it seems like something just popped.

Driving the car results in no juice getting to the coil and it runs poorly. The fuel pressure dwindles away as the pump slows down, too. I've driven it about 75 miles like this, in one trip, at night, and it never killed the battery enough to kill the car. The lights were dim and the fuel pressure was getting scary-low but the car never just stopped.

I've swapped in a different horn-relay/junction block, a new remote solenoid, all the fuses look good, etc. With the hot lead off the battery the multimeter reads only about 7.4 volts from the wire terminal.

Another thing I noticed, which may be okay, is at the ignition-hot wire from the harness to the alternator pigtail: The white (?) wire from the plug in the alternator is wired to the ignition-hot wire from the stock external regulator plug. (As per Painless instructions). Just testing with a test light I noticed that the light is pretty bright/hot before the two wires are connected. Once connected the current in that line only dimly lights the test light. I've tested it enough to verify it's not the connection from the tester. It's just an anomoly I noted. I don't know if this is a clue or not.

Any ideas? What other tests would you use to diagnose this?