Hello Pete,

Straight off, make sure that all connections to the starter and
battery are in good order. Check the braided lead between the negative
battery terminal and the firewall for any corrosion or fraying. My
connections all looked clean and bright, but there was corrosion inside
one of crimped connectors on one of the starter leads. The loop that bolted to the starter post looked good, but the connector wasn't making good, or steady
contact with the positive battery lead that it was crimped round.
Took a couple of starters and battery changes to figure that one out.

Ensure that there is a good ground strap between the
chassis and engine block. These are also originally a braided affair, and
tend to weaken with age. When this happens, the choke cable and/or
throttle linkage becomes the primary ground, resulting in a less than
adequate voltage supply to the starter.

Check the condition of the teeth on the starter motor drive, and
the flywheel ring gear. There might be a burred tooth on either, or a few
knackered teeth on the ring. The only remedy for wear is replacement.

You mention that the starter has been rebuilt, but there's
still a possibility that the solenoid has a burnt area on the disc that
supplies power to the actual starter motor once the solenoid has been
energized. This too can cause intermittent starter operation. Also, check
that the connection between the solenoid and the starter is in good nick.

Best,
Bearman