<VV> Starter grinds unless switch is turned firmly?

[Ignacio Valdes]

Hi, My 1964 Corvair Monza Coupe 'Lisa' has developed an issue with the
starter. If I turn the switch lightly the starter does not engage very well
and it grinds. If I turn it quickly and firmly it engages and does not
grind. But I have to 'think about it' when I turn the switch or it grinds.
What is going on? It did not do this before. Solenoid is Clark's replaced
about a year ago. Starter was rebuilt about a year ago.

-- IV