A growler checks for unwanted circuits in the armature, ones that create induced current that opposes the rotation of the armature. When an oscillating magnetic field is induced into the armature core by the growler, if there's a copper-to-copper "short" in the windings, a hacksaw blade or other thin steel strip will vibrate when it's passed over the armature. A "ground" is an unwanted copper-to-steel circuit, and it's found by touching a test light to the commutator and the shaft. A final test is for continuity, done by touching the test light probes to adjacent pairs of commutator bars all the way around. If the armature passes all these tests, it's good. I'd recommend looking for a loose or corroded grounded brush holder that's making intermittent contact with the end frame or housing, which would explain the good and bad performance at times. If somebody has done a really good job of painting a "rebuilt" starter, that can also interfere with current flow between the components, and/or the bell housing.
Jerry
