If the starter will sometimes engage and sometimes just spin when you turn the key is this a sign of a defective starter?

This sounds like there are some teeth missing on the flywheel. If the bad spot is positioned over the starter gears, the starter will just spin but if it catches on a good part, it will turn the engine over fine.

Just what the previous answer said. You have some teeth missing on your flywheel. There is probably nothing wrong with any of your starters.

If you have a manual tranny, just put the car in gear and try to push it forward until you fill the flyweel engage with the starter. The car should start then. My 1986 Ford aerostar had the same porblem as stated above and this was an easy solution until I had the time and money to fix it correctly.