It may be that the pump internals are worn out. With the hours on your machine, it's not unlikely. You might want to check the pump pressure and see if it's within specs. If it's not, a rebuild or replacement may be necessary.
Have you inspected the relief valve? Sometimes these can get stuck open or be under too much pressure. Checking or replacing this could potentially solve your issue.
One thing to consider is your torque converter. If it's failing, it could also cause similar issues. Check if the converter is delivering the necessary torque to keep the systems running smoothly.
Did you also drain and flush the whole hydraulic system when you changed the filters? Sometimes sediments build up and can cause blockages, which affect the overall performance.
Considering the age of the machine, it may be worth checking all the seals and gaskets as well. Over time, these can degrade and cause internal leakage, leading to pressure drops.