well, sounds like a corroded ground wire... check both starter wires for corrosion under the insulation, you may have to crop bacl, solder in some new wire and the connectors... Could be the starter going....In which case the hitachi mini starter (hi tork) is the starter of choice. Also would be smart to add an extra ground wire from motor to the chassis....I used a 18" battery cable bolted up to the pass side fender well, and a bolt on the timing cover... now bear in mind I live in Fla, not the rust belt so corrosion isnt an issue here, but things get old....
