quick alternator question

[WRMZ71]

My generator light in my truck won't go off. I've never had a problem starting it. Multimeter on the battery not running says 12.6 volts and when running i rev it up and ten it goes to 14.6 volts. It seems everything is working but that damn light won't go out. Every once in a while while driving down the road my volt gauge inside will jump up to like 18 volts and it will be there for awhile but then go back down to the normal 13-14 volts. The lights get dim to when I"m sitting idling. Does anybody know if this is a regulator thingy or an alternator thing? It's a 71 chevy with the external regulator. I tried grounding out the tab on the back of the alternator to run it unregulated and see how much i get but i can't find it. Any help would be greatly appreciated and thanks in advance.