[aussie merc]

its certainly odd but check the driving gear within the pump itself and make sure its positively locked to the shaft [keyway, dimple, weld, spline it doesn't matter ] but interference fit wont work for long . Cummins had major dramas with this in its Bigcam 3 series the pump gears would slip under load and oil pressure would disappear the fix was a new repair kit for the oil pump with keyed shaft and gear