I don't know if this is your problem but I had a similar thing happen, after much puzzling I discovered that one of my speaker terminals had been bent and the speaker was shorting out. The only clue I had was the weird behaviour of my CD player and a flat battery.(posted 8090 days ago)My battery light accasionally comes on when the alternator apparently fails but I have found that if I thump the alternator housing with a hammer a few times to jar it while it's running (ok, a bit rough)and maybe rev the engine it seems to free up whatever is inside and it works again fine. Thise fix lasts 6 - 12 mnths and has always worked.
Hope this helps.
Regards, Daryl Talbot
PS I have an '84 900 Turbo