I put a new alternator in my 89 Tempo last year, with the correct belt
tension.  These are a pain but I had my veteran-mechanic neighbor verify
that it was fine the other day when I was doing other work.  The voltage
began oscillating at idle (all lights pulsing) and it would go away once
off-idle.  Now it happens all the time, to the point that the spark
voltage goes up and down and you can feel the car cut out a little, to
the exact rhythm of the lights dimming.

Did I get a POS alternator?  Does the 89 have an external regulator?
The Haynes manual (sp?) shows that it's internal, and so I'd have to
just replace the freaking alternator again.  It's a 2.3.

If I have to replace the alt., any suggestions on aftermarket brands to
avoid or ask for?

Thanks.