Having a wiring diagram is very helpful. Ok, it's an alternator with external electronics (rectifiers and regulator). That means that the external leads from the alternator are AC, and current carrying. With this setup, the leads leaving the alternator and any connections in the leads are many times themselves the issue, but if not, any problems with the charging circuit are almost always the regulator (which is actually the rectifier and regulator). With the leads disconnected, they will measure up to 50 volts depending on the speed that the alternator is turned.

So check for broken/intermittent wires, overheated plugs, and that sort of thing. If you don't turn up an issue with the wiring, my bet is on the regulator.

Hope that helps,
Tom