I also have a 2000 F150 and It's only once in a while but it does the same as you are speaking of. I am gonna inspect the Flywheel myself wandering if it is a glitch in their starters? since it's happening to many others.

 

I have had the same problem with my 2000 F150 5.4l I replaced the flywheel and starter and cables and battery last year. It worked ok for a few months and now it's back. I am going to put it up on a lift and remove the inspection plate and see if it engages all the way. Also this is the third starter in 10 yrs. Does anyone have the right fix.

 

I also have this problem once or twice a month on my 98 f250 LD. Currently I just release the key and it always fully engages on the 2nd try. Before I go through and needlessly start replacing everything, I am hoping someone out there knows what is the MOST prevalent cause(s) based on their fix/experience.
I only have 36,000 mile on it, and plan on keeping my truck for 10 more years. I prefer to fix it now, rather than in the grocery store parking lot with ice cream melting or at lowes with 40 sheets of drywall loaded, and a crew waiting.

Thanks in advance for your help!