Oh, sorry...thought only the HID was pulling the voltage down. Saw in your earlier thread that lights and AC did the same.
Cables are always something to check...but I think you already checked for voltage drop...
"running, lights, radio,ac/hi, HID fogs 12.23v
Crawled under, took same reading at alternator got 12.4 volts."
Well, I'm thinking it could be one of the following...
1. It still could be your alternator...you said it was full field tested...not sure of the details of the test preformed by your mechanic, but it should be 'load' tested. Why load tested...well, for example, a 'bad' battery can give a voltage reading of 12.5V, which seems good, but under a load, it could drop well below 9V.
2. It is your PCM...it is not sensing the added electrical load and doesn't 'tell' the alternator to compensate.
3. Or as others said in this thread...the wiring...not just the main cables, but the wires that go from the PCM to the alternator...reason is...doesn't seem to be acting consistent from what you described.
Sorry can't be more help...but it is hard to diagnose something like this without being there.
But in all honestly, I'm tending to believe it is your alternator...even if it has been rebuilt...