[MagmaJct]

I'll take a stab as this! The problem sounds so much like worn brushes to me. How did Autozone perform their test? Will there test work on an intermittent problem? Often times when DC motor brushes fail the problem will be intermittent.

If the problem is NOT the starter nor the solenoid, then the problem is a huge voltage drop somewhere in the circuit. Do the headlights dim considerably during the "no start" condition when the starter is engaged?

There are some test you can do, but they will require a helper. Grab your best multimeter on the DC volt setting. While the starter is engaged and it's not working, measure the voltage from the battery positive terminal to the positive terminal on the solenoid.

What we are doing is measuring the *voltage drop* across the positive leg of the circuit, not the voltage across the battery. The voltage drop shouldn't be more than 1 - 2 volts. We shouldn't see battery (12) voltage!

If we see a low voltage drop on the positive leg of the circuit, next we need to check the ground or negative leg of the circuit. Again, put your best meter on the voltage DC setting from the negative terminal of the battery and the negative (grounded) terminal/body of the starter. Again, the voltage drop should be small. If it's huge, there's your problem.

Once the voltage drop (if there is one) is found to be in either the positive leg or the negative leg of the circuit we can narrow down the fault using the same method.