Temporarily connect a jumper cable from the battery ground terminal to the bellhousing or any place on the engine, then try to crank the engine. It sounds like you're losing a ground from the engine to the chassis. There is SUPPOSED to be a ground strap from the bellhousing to the body and/or frame.
Most of us run a long battery ground cable all the way to one of the bolts that attach the starter, or at least to a bellhousing attaching bolt. Then run a smaller ground strap to the body to get a good circuit for lights, heater motor, etc.
Jerry
