If it's in spec clean the pickup screen and throw it back in torqued to spec. The pumps are capable of ridiculous pressures when the oil is cold, and the only failures (outside of those related to abuse) have been from sticking pressure relief valves causing the oil filter or oil cooler seals to blow out on cold mornings. I expect that not following oil change intervals and/or not using a diesel rated oil may contribute to this condition.
I usually run the pump in some varsol - you can generally get a good feel for how effective the pump is, and it cleans out the soot - and reinstall it.
Low oil pressure on high mileage engines is often related to bearing wear - especially main and intermediate shaft bearings. I had one engine where the intermediate shaft bearings had both all but disintegrated, and there was no oil pressure at the head, and only about 2 pounds at the oil filter at idle when the engine was warm.