It is obviously somewhere between the battery and the starter, probably in
the big cables and their CONNECTORS. I'd start by removing both cable
connectors at the battery and render all contact surfaces clean = metallic in
appearance. Then make sure the cable is good and tight in the connector.
If it is bolted on, unbolt it and clean faying surfaces. Be sure your engine-
to-frame ground cable gets the appropriate scrutiny also. Probably down near
the exhaust pipe on the underside, bolted to the bellhousing. Check the far
ends of the battery cables also, where they bolt to whatever they bolt to and
make sure they are TIGHT.

I have had a number of strange problems with these high-amperage connectors,
particularly weird ones where a nonconductive film has developed in seconds
between the battery and the cable. I don't completely understand everything
I know about these things.
--
George Downs Bartlesville, Heart of the USA!