Thanks Everyone for the suggestions, I have resolved the problem.
Turns out that my custom thermostat housing was to blame.
entered the housing, then went straight out the exhaust
. Even when the thermostat opened, this was the easiest path, so there was very little circulation throught the head
A modification to the housing restricted flow between the inlet and the outlet to exhaust elbow
(made it 50% smaller hole than 'prefered path'). This 'encourages' correct flow path, but still allows it to go out the exhaust while thermostat is closed. The new housing opened everything up too much, and did not direct the flow.
Now sits at 62-67 degrees under load - much happier about that than 80+.