This is the flow sequence of the starter system.

1. 12v at Battery to...
2. 12v after 50A Fuse in main fuse box in engine bay to...
3. 12v on Battary side of Ignition.
4. 12v from Ignition switch to the starter (small wire on starter) with the ignition in the start position.
5. 12v straight from the battery to starter (big wire on starter).
6. Check the ground from the tranny bracket to the chassis for good contact and no corrosion.

That's it! If you had the starter, the battery and the fuses checked...then I'm leaning towards a ignition switch going bad.