I'm getting some good electrical advice, but I did jumper a fresh battery
directly to the starter. I've also, at this point replaced the starter, so
I'm ruling out electrical.

I also replaced the bushing.

What else to check?

>> >
>> > Symptoms:
>> > Turns over very slowly, sometimes stopping (or catching?)
>> > Runs fine when started.
>> > Battery voltage looks good. Jumping doesn't help.
>> > Voltage at starter matches battery.
>> > Turns over slowly with:
>> >    - spark plugs removed.
>> >    - fresh battery jumpered directly to starter.
>> >    - WITH NEW STARTER INSTALLED!
>> >