Self dimming headlights - faulty alternator

This feature has installed itself in my car :)

Driving it in the dark yesterday for the first time since long, I experienced following:

At random the head lights AND the speedo light AND the warning light for flashers (if in use) lights up significantly. Like switching from 12 to 15V... It randomly switches back to the 'normal' level. Or the other way around (normal level is not that much, but never had been better). Happened approximately 15 times on a trip of 40 minutes.

I fitted a temporary voltage gauge and made a test drive.

Most of te time the alternator gives a proper voltage:

Immediately after starting it went thru the roof, up to 19V. 5 seconds later to a steady 13.6 volt at cruising speed and 13.4 at idling. During the 10 minutes trip it went up to 17+ volt 2 times for a couple of seconds:

I took it apart, but could not find any fault. Actually, the only thing i could check were the brushes: these seemed to be OK.

I bought myself a new alternator (made in China, I know.... ).

It is giving me a steady 14.2 Volt (driving) and 14.1 at idle 

Learned from several forums these new alternators are not expected to survive for long, I intent to repair the old one for spare. I guess the small regulator (?) box in the alternator is broken, see picture below.

Got a tip for a complete overhaul set: 
  http://www.ebay.co.uk/itm/LUCAS-ALTERNA ... 2a25ef3d20

Something for next winter....