Kubota KH90 loosing hydraulic power to tracks when warmed up?

I own an old Kubota KH90 excavator, everything runs fine for about 30 mins, when there is a slight loss of power when turning on the tracks.This gradually gets worse and it affects all other movements with loss of power. The hyd oil tank feels very hot. I have changed both suction and return filters which were partially blocked but the fault still accurs after 30 mins when the machine is warmed up. Any advice would be much appreciated