my 88 vanagon did something similar, and when tested, the battery showed
a dead cell (don't they ALWAYS say that?) ... but with a new battery, it
was a brand new machine. :)

what the fellow told me was that the plates in the battery tend to flake off
some lead and that this lead will accumulate in the bottom of the battery.
there is even a special area under the plates designed to accumulate this
stuff. anyway, the stuff will eventually (moreso the older the battery is)
pile up enough to short out a couple of plates, and you'll start having
problems. it might go away for a while (if the pile is shaken flatter by
bumpy driving), but it will come back.

and it seems to be a hit-or-miss thing with new batteries, even. i once went
through three Sears Diehards in six months ... but that last one was still on
the car when i sold it five years later.

by the way, Wal-Mart has Champion batteries with better warranties and cheaper
prices (than Sears). just a thought.

joel