hard starting diesel when hot

G'day all,
has anyone had problems starting a diesel when it is at running temp. The symptoms are that of a flat battery (slow cranking). The vehicle in question has dual batteries -both have been checked and found to be ok.
When it is cold, it starts first kick even in snow /ice. It starts when jumped off a booster pak or 4wd but not when jumped off a car. This to me is a little strange when both batts are good. It is a 1HZ motor.