87 5000CS Avant with electrical issue

[Stephen Sherman]

We had a cold snap the last few days (-10F or so at night, around 0F 
during the day), and our old, trusty 87 Avant did not handle it well.  
Not the expected problems, won't crank or start; those things were fine. 
Rather it was an intermittent loss of electrical power: headlights dim, 
dash lights dim, battery icon comes on, anti-lock switches itself off. 
Worse at low RPM. It seemed at its worse, that the voltage was getting 
so low that the ECU and engine was starting to not work. I ended up 
being able to drive it home by staying in low gear to keep the RPMs up, 
and rev'ing the engine at stops.

Did a quick check, and voltage at the battery is ~13.7v with the engine 
at 2000rpm (this with the very low battery right when I got home). Seems 
within the range of a working alternator and regulator. Maybe a bit low.

So my question is (of course) what is causing this. I remember reading 
on the list about the union/splice in the battery line, somewhere on the 
passenger foot well. (details on exactly where to find this would be 
appreciated), but it seems odd that that connection could be causing 
this problem, yet the car would still be able to start just fine in cold 
weather. Same about thoughts on the alternator going, since it does seem 
to recover at higher rpms.

Any thoughts on what might be the problem here, or what to check out?

TIA