[Richard]

could be that the starter is due for rebuild. Brush or armature problems would explain hit or miss spinning and your voltagwe observations. Sounds like the solenoid is pulling in, but the starter motor is not spinning. Is that right? Then occasionally the starter motor will spin.

On the occasions where the motor doesn't spin, does a love tap with a hammer cause it to turn over? Showing my age here, too many years of living with raggedy automobiles.