thanks for the update
And just an FYI: You can have voltage present but the corrosion will inhibit current flow. Both are required for the circuit to function properly. The flakey connections at the fuse box were likely drawing variable amounts of current causing the voltage to vary as well. This would result in the issue as the VR and Alternator were tying to respond to the changes. might be a good idea toc oat that FB contacts with a conductive coating to prevent further corrosion