If there's an open circuit in one of the armature windings that connect to the commutator bars at the end of the armature, when that circuit stops under one of the brushes the armature won't turn when the stomp switch is pushed. There's about a 1:30 chance of that happening, so it will be intermittent. Take the back off the starter and look for a burned commutator bar, or even badly worn brushes that don't make good contact with the armature.
Jerry
