the volt reg is screwed to the back of the alternator, remove that and check the brushes are a good length and roughly equal. then look at the unit ands you'll notice a ring around one of the screw holes, give this a buff up and the area on the alternator it touches so as to give good contact. last of all, if ytou look in the hole the voltreg came from, you'll spot a metal prong, clean this up and the surface on the volt reg that it touches.
had exactly your problem on a mates 16v, did all the above and it was sorted
