Voltage reg?

[Cody Forbes]

I haven't seen a failure mode quite like this before so I shal employ  
the collective wisdom. Seems like a voltage regulator failing, but  
could maybe be bigger.

So I'm cruising down the road in my '87 5ktq taking the family to the  
mall. At one point I noticed the tach fluctuate a little. Not quite  
like the old bouncing tach syndrome, just a lazy ~300rpm occillation  
while the engine was definately steady. I pulled up the volt meter in  
the climate control head and noticed that we were down to 10.3v. I  
immediately turned off the A/C and the voltage came quickly up to  
about 12.5v. This car has never been a great charger for the 4 years  
I've known it and two that Ive owned it, usually 12.8-13.0 daytime and  
low 12's at night with lights and everything going. Anyways, when I  
stopped at the next light the voltage very quickly rose to 14.0v with  
the engine at idle. As soon as I raised the revs to pull away it  
dropped back to 12.5v. We were more than halfway to our destination so  
I decided to motor on. On the highway voltage fluctuated 12.3-12.9.  
Back off the highway I noticed the voltage fluctuating with the  
cooling fan as it should, but it was swinging from about 12.3 with the  
fan on to near 13.8 when it turned off.

-Cody (mobile)