For you electrical gurus in the audience...

Sometimes one of the windings in the armature can have a cold solder at the junction of the commutator or a crack in the wire that will function until it gets warm where in it opens the crack from expansion. Generally all this will show up on an ohms check. A crack or cold solder will show high resistance. Meggers can also work for insulation testing. Insulation does break down over time. Growlers just do the insulation check in a different manner.

Field junctions can also have cold solders so it's important to check the fields as well.
 
I resolder every one, the key is to have a large enough iron to heat it instantly, I have a 600 watt iron that does starter windings quick, but if you use it too long you can get sunburnt
 
Its nice to have a big-*** iron hanging around. When you plug them in you can hear the electric meter spinning and the wall its mounted to shaking.
 
One of the greatest advances of recent decades for modern vehicles is that starters and alternators are so cheap new, that it is not worth reconditioning them. Every reconditioned unit I ever bought had something wrong with it that showed up in some remote location ...
 
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