Go check all your electrical connections. Check them at the starter, alternator and battery. I bet you will find either a broken wire, loose connection, or corrosion. Chances are good that it won't be a relay or the solenoid(especially since your saying you had the starter checked) but of course it's still possible.
If it was me I would take off all the connections and clean all the terminals and wires. Tug on the ends of the wires and feel the shrink tube on the end for anything flimsy feeling... Indicates corrosion. Make sure you reconnect everything and all is tight.
Cover all the basics first before spending money on parts.
Good luck my friend
2005 DSG ranger 4x4, 4.0L 5spd.
Dual magnaflow exhaust, 6000k HIDs, kenwood deck with 2 10" alpine subs
33x12.5 wrangler mt/r kevlars on 15x8 ultra nomads, shackles and cranked t-bars!!