Starter Engagement

[RBuhr69]

Need some help trying ot figure out an issue when starting the car (69 Camaro).

Occasionally when I go to start the car 383 SBC, the starter does not engage and hits the flyweel and makes the loud screetching noise where the pinion does not engage. It only does it once in awhile and not dependent whether it is is hot or cold when starting.

Things I have done....

Replaced starter with three different ones, all new..
Shimmed starter several different ways
Replaced flyweel/flex plate.
Brand new Optima Red Top
All new wiring from battery to starter, and ground cable.
Ground cable is connected to block

I did call MSD to consult with tech line, and they said it sounded like a voltage issue due to it being sporadic. So my question is, could it be the igniton switch to the cylenoid causing this? My thinking is the switch only activates the cylenoid and the voltage actually comes ffrom the battery.

Any thoughts, ideas?