[brshap]

Just to circle back on this thread in case it can help someone in the future, the culprit was.... the battery positive cable (at the starter)!

Despite cleaning it with a wire brush and no visible corrosion it must have had enough resistance to not deliver enough amperage to the starter. This is explains the inconsistent voltage drops. A clean up with some fine grit sand paper did the trick.

Thanks to everyone for your help.