well, starters can be pesky .... as Lisa recently found.  I'm also having
difficulty with mine now.  Replaced my solenoid (was not working) bench
tested the starter and works great! .... Installed it in car and again
experience that it won't turn over every time... once in 3 or 4 instead.  
Removed starter again, bench tested... works well.  Reinstalled starter and
same problem... the pinion gear wont engage every time... I know now that the
bendix does work though.... anyone have any ideas??? Quite frustrated here

- cheers, guenter

ps  flwheel and pinion gears look fine... no obvious wear marks, no iron
   shavings etc...