I'm pretty baffled. My 85' Westfalia waterboxer has been turning over
slowly since I bought it several years ago, but it has finally got to the
point of stranding me. It is behaving like a low battery, but I think I
have ruled that out.

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!

What am I missing?

Thanks,

Ken in Oregon.