11.7V where? At FiTech? We have no idea where that is powered. Seems like posts from 67Dart273 and others suggest ways to test your system, have you done that?
Alternator needs to spin, it takes power, so a slipping belt will not generate much output.
The regulator needs a reliable voltage, it decides from that voltage, to energize the field or not. More field supplied, more output of alternator. But if the voltage that supplies the regulator cannot supply the necessary field current, due to very poor connections the alternator wimps out. Poor connections from bulkhead connectors, ignition switch IGN1, visible link.... Moderate connection problems, result in over voltage, but more severe problem, starves the field.
A relay can be used to bypass the problem, and 273Dart273 has a post on that in the forum.