I had this on a 1996 Express van. Fix was simple yet took forever to find.

The hot wire on the battery, at least on the van, ran from the positive pole to a fuse box sort of thing on the other side of the engine compartment. This was more a box full of fuse able links than a real fuse box. Where the wire connected to this box was a single pole with a nut that screwed down on top to hold the wire.

The nut had come loose and the wire was not making good contact. I removed the wire, cleaned it and the pole, stuck it back together and the trouble vanished.

Hopefully this is all you are dealing with.