Loosing most of voltage after attempting to crank

Loosing most of voltage after attempting to crank O7 Ranger won't crank or start. New starter & battery. Checked that it had 12 volts at starter. 1st Attempt to turn key, lights are all on & it clicks once & nothing. Release key. 2nd Attempt, no click, no lights, no headlights, nothing! Check voltage back at starter again, dropped to 2.86. Wait 30 seconds or so, dash & headlights back, along with 12 volts at starter. Any help or suggestions? Thanks!