Starter - troubleshooting question

[Dave C]

Patient is an '89 100
Starter was replaced w/ a rebuilt about 4-5 years ago.

Now the starter is displaying an intermittent problem, although sometimes
it works fine.
Other times it will start to crank and then quit with an odd whirring noise
... sounds to me like the starter dis-engages from the flywheel.
Usually the next turn of the key it is OK and the engine fires right up.
Battery voltage is good (12.7 volts), and the under hood jumper post reads
the same.

Does this mean ...
A. Something in the starter needs to be lubricated? (How and what?)
B. Starter needs to be replaced?
C. Something else?  Maybe the ignition switch?

As always, any help is greatly appreciated.
----------------
Dave C.