Kubota KX41 bogging when hydraulics are activated.

Hi team. I'm trying to find out why revs drop from 3000RPM on idle to 1800RPM when hydraulics are activated. Bogging happens on any hydraulic action.
Here's what I've done so far:
- emptied and cleaned fuel tank (had water, rust and gunk)
- replaced fuel filters (clogged with water and gunk from tank)
- replaced fuel lines (lines were brittle) - had injector pump and injectors tested - tested ok
- bled injector lines
- replaced engine oil and oil filter
- replaced thermostat and coolant (thermostat was stuck open)
- air filter had already been replaced - exhaust is clear
- checked governor spring, forks and flyweight. Worn but nothing broken or missing
- cleaned and flushed hydraulic tank, replaced both filters (both filters were very clogged), filled with new oil.