1988 be fox gl wagon electrical issues

Hey guys. I have a 1988 Volkswagen Fox GL wagon.
Recently it have been giving me some electrical issues.
Suddenly it wasn't able to keep up a charge for all running
electrical devices unless I was actively revving it. I first
noticed this because my aftermarket stereo would shut off
when there wasn't enough power. I replaced the alternator
and nothing changed. Then I realized the puppies were
oily and the belt was probably slipping. So I cleaned those
off, tightened the belt again, and voila. It ran wonderfully,
for half a day. And then went back to doing the same
thing.  I had big the alternator and battery professionally
checked and I'm told they are both in excellent condition.
Both being new or fairly new.   I have also noticed that
that if I have the heater blower motor off that it has
enough juice while idling to keep things going. Even
though the headlights still dim significantly until I etc it up
again. So suddenly, the newest symptom to go with this is
when I start it after it has been sitting in the cold all night (I
live in Alaska and it's winter right now) it will start just fine
but the battery light in the dashboard will stay on and any
devices running will run the battery down until the
alternator eventually takes over. And in that second that
the altornayor eventually takes over the radio will turn
back on, the headlights will brighten, and in general, any
electronic that was draining the battery will start working
again.  This issue with the battery light staying on during a
cold start has been going on for about two weeks now
and every time it happens it takes longer for the alternator
to take over (originally being about 30 seconds the first
time, now taking up to maybe 4 minutes).  This makes no
sense to me. If you guys can give me any advice on what
might be causing this or any of these issues I would be
more than grateful. I love my Fox and I want to get it back
to its dependable running condition that I have been used
to. Thanks guys!