So I still don't know why the alternator passed its bench test, but I picked
up a second hand voltage regulator from a very nice chap online and swapped
it in and all is well. The brushes on my old one were spent, so I suspcect
that was the issue. I felt a little funny about getting a used one, but for
$12 instead of ~$115 I figured it was worth a shot... not that I could even
get one around here (didn't try the dealer... got too scared). Thanks for
all for your input.