About 2 weeks ago I was driving and my stereo and dashlights went out.
About 4 minutes later the check engine light came on and my car started
running very rough and was idling around 500 RPM.  I pulled over to the
side of the road and was giving it gas and after a couple of minutes
the stereo came back on and everything was fine.  Then it didnt't
happen again until about a week after.

After a week it would happen on and off, but when it happened, it would
only last about 5 seconds (the stereo would go out then 5 seconds later
it would work).  This happened on and off for about 3 days.  Now it
happened again, but it stays dead.

I get a boost and it works for about a minute, then dies.

Is this the alternator?  I am unsure because I don't know if when the
alternator dies, it can work then stop working then work then stop
working.  The only other thing I could think it is is a short, but I
hope not.

Does anyone have any experience with alternators or this problem that
they could give me some feedback with?

Oh yeah, it is a 1989 900S.

Thanks

Ed