I had Geronimo's original starter do this - it was the 2 plastic pieces that the arm pivot pin goes through. They had deformed over the years, and it would intermittently cause the arm to not move through the whole range of motion. My friend at the alternator shop had a junk starter he pulled the parts out of to fix it, but it finally got so bad the housing was shot, so I replaced it with an AutoZone starter to lifetime warrantee - they suck, but they hand you a new one when they fail - took me 5 to get one that mostly works.