Pull it apart and thoroughly clean it out. Repack all of the drive gears with fresh grease and polish up the pick ups on the armature so you get a good electrical contact. Make sure the brushes aren't gummed up in their housings and aren't worn excessively down. Once a starter motor starts to glitch like this, it is letting you know it's not going to last much longer without intervention. I have resurrected a few starter motors and they have a tendency to not work once you've reassembled them. Persevere with testing and after a few tries they will kick over reliably every shot.
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