Lucy is on her third alternator this year. A few months ago the factory piece started putting out close to 16v. Had it rebuilt at a reputed alternator/starter place. Two weeks later it failed and stranded my brother an hour from home. [partially his fault. He thinks that the instrumentation are for decoration and ignores them] Had to have a local garage change it out with a new Bosch unit. (100 for the rebuild, 300 for the Bosch installed and they kept the core).

Now, the charging system is acting weird. When I first start her up, the voltage needle goes to where it should, around where I believe 13.5-14v to be. After a bit though, the needle goes down to just above 12v, and often will sit right at 12v. Now, never has the needle actually dipped below 12v, and this has gone on for a couple weeks; the car still starts and everything works. Manually bringing the idle up to 1200-1500 brightens the headlights and cluster backlighting back to normal. Spirited driving also seems to give the voltage a kick in the pants. Haven't been able to actually check output as my volt/ohmmeter is dead.

Belt is nice and snug against the pulley, and both the +output stalk and control connector are secure.
What's going on?

 

You won't really know until you test the alternator voltage output.

 

Simple you got another bad alt. It's common on these old cars. Your first alt had a bad voltage regulator (it's inside the alt) while the second one has weak output. Though it might be good enough to use if you don't use headlights/etc. too often or if you keep the rpms up with a lot of freeway driving. Not that you should need to. The alternator should be covered under warranty and any auto parts store will test it for free.

Changing the alt is very easy. You might want to grab a Haynes manual and learn to do it yourself. Especially if you end up with yet another bad alt. $300 is a total ripoff for a $100 alternator and 15 minutes of labor. You can also search the forums for information on upgrading to an FD alternator or other alternator so you don't get 25 year old alts that sometimes have something different go wrong shortly after being rebuilt (including the "new" reman you got).