My 6 month old Leece Neville suddenly started acting up with large voltage swings. In a fit of stupidity I chose to ignore this for a while till it was more convenient to check. I found the positive stud loose....really loose....actually the stud was burned in two and hanging by a thread.
Upon disassembly I found that the studs are nothing more than common carriage bolts that must be kept tight in order to maintain good contact with the rectifier blocks. In this case it's a newer style alternator with a one piece rectifier and a corner of the large aluminum casting was melted away by arcing from the loose stud. I purchased a new rectifier (50.00), R&R, no output, dead as a hammer, aw shiucks. Then I took a paperclip, flashed the field, (thought you didn't have to do that with alternators only generators but not so) and it's back to a steady 14V.
You might keep an eye on those studs. The bolt furthest down on the stud that tightens against the plastic brush housing is the one that must be kept good and tight.
Chuck