Make sure the battery terminals are clean and tight too. My fathers van would sit a lot, and he would pull the battery out. Often times, he would just place the battery in place and not tighten the terminals, just push them on "snug". His has a pigtail under the nut on the NEG terminal which I believe is logic ground an it would do all sorts of strange things. I believe that sometimes the wire also corrodes back under the insulation by the terminals and may look like a solid connection, but isn't.