Dying battery

[Leah and David Mueller <muellerl@cadvision.com>]

This morning, after sitting for three days, our 86 5000s didn't did not want
to even attempt starting.  The low/high voltage sign came up on the screen.
After boosting the car started up right away. The car did fine for the rest
of the day.  

A couple of times in the past the red triangle waring light has come on
briefly as I pulled out of the drive and then gone out in few seconds as I
get going.

I have had occasional very long cranking times to get the car started.  

Anyone had this before?  When running, I measured 13.5 volts at the battery.
A intermitant bad alternator? or battery?

Thanks

David