I'd like you to try a test. I have a slight hunch but I want to rule this out. First, the stator is the big round thing inside your alt that has copper wire wrapped in numerous places. This is one of the main components of generating the voltage. The stator is the immobile portion. It is possible that one of the diodes have partially failed alowing current to either work in reverse or generate AC on the output side. Unplug the stator wire, and test your DC voltage while engine is running. Now, remove leads and switch meter over to AC voltage and test. You should NOT get any more than 0.5V AC. If you do, one of the diodes are bad. And that may also be causing your voltage to be locked, but fails when stator is connected.