Since this is a gear machine, I will assume it is the engine that is loosing power and not the hydraulics.
How many hours?
Fuel filter changed?
Do you have a way to check temperature accurately?
Try loosening the fuel cap and see if power restores.
Does the fuel bowl remain full?
Worst case, the Injection Pump is getting worn and needs to be rebuilt.
First thought would be clogged fuel filter or clogged line from tank. Remove the filter and see if you get a full flow from the tank. If you only get a trickle, there is a clog somewhere. Also don't forget the air filter.
Sorry I didn't explain better. Engine power is what I'm losing. It will run strong for about a hour. Then it will start slowing down more and more the longer it's ran and eventually kill. I can let it sit for a hour and it will run like it should. No temp gauge and it has new fuel filter and the lines have good flow.
~35+ year old machine? It could be quite a few things. Fuel delivery restriction is a good possibility. Fuel cap vent, fuel tank outlet restriction, fuel hose deterioration / debris blocking filter housing fittings.
I'd first remove the fuel cap and run it for a while for testing. Then disconnect the fuel line at the primary filter and check for adequate flow.
If it runs well when cold the injectors and compression are probably fine but don't overlook engine operating temperature and possible overheating.