electrical weirdness

[LOACESQ <LOACESQ@aol.com>]

The Audi gods are at it again.  Just when things were running smoothly . . . 

Here are the conditions and symptoms:

1. '87 5kcst with some history of weak charging/battery discharge, but no
other history of electrical problems;

2. while running (happens more at idle and low speeds, but has happened at
highway speed);

3.  each time the radiator fan goes on; 

4.  the seatbelt warning light flutters briefly (accompanied by weak bell),
and sometimes the ABS OFF light flutters on as well;

5.  if the radio is on, the speakers will cut out briefly at the same time
also, but the radio itself still has power, as indicated by the display.

6.  no other problems apparent.  While the car is running, the alternator
seems to have no (other) problem supporting the full electrical load.

I've looked at the wiring diagrams as well as the actual wiring and can't
figure any of it out.  Anybody have a clue?  Please?

Voltage regulator?
crossed wires/shorting?
alternator?
relay?
demonic possession?

TIA for any tips!
adam