Vic --
It's so hard to diagnose this stuff over the internet...
Is this the same starter you've had all along? To me it "sounds" weak... not sure if that would be because your battery is low or the starter bendix is weak - or the wiring is too light of gauge etc....
Does your battery cable connect to the starter?
Does your alternator connect there too?
What's the voltage of your battery?
The shimming is more about the retraction of the starter motor - and to have enough clearance etc... not the actual engagement. You're having an engagement issue. Of course this could cause an engagement issue if the starter gear was too tight to the flywheel - but you seem to understand that part and I'd assume you have it shimmed just fine.
Did you put a Ford starter on your Chevy??
