As John says above check the earths first. I had similar a few years ago on my 4/4. As a test I used a long booster/jump start cable from the battery neg to the starter mount, bypassing the earth/chassis route. Danger!,,, connect the starter end first then battery! The car started ok. I then replaced the earth cables. My car had the battery near the rear axle so two short earth cables needed. Cleaned the contact areas, new stainless bolts and petroleum jelly smeared over for protection.
