My truck started doing this really bad for a while. It got so bad it took several tries to get the starter to engage before spinning. I thought I might be developing a bad spot on the fly wheel and would roll the truck two or three feet forward (in gear) to engage at a new spot with no improvement. I decided the starter was spinning before the drive gear was fully extended and engaged.
I backed the stomp starter pedal out to ensure I was getting full extension on the drive gear. Then I tweaked the stomp starter mechanism on the firewall side so that it would engage the electrical switch and spin the motor at the last part of pedal depression.
This was an almost 100% successful fix and the only time it does it now is right after I tell someone "It starts right up every time" *GRIND!!*, or starting it up to leave a car show *GRI-I-I-ND ! !*
