My ten cents worth:
You stated that you have an A65 oil pump body fitted, is it the late cast iron higher capacity type or are your pumps all mazak earlier type. From memory the late pumps had "O" ring sealing the shaft inside the pump to stop oil transfering between the feed and scavange gears inside the pump if the pump body is worn in the shaft area, is the oil leaking between the 2 chambers when hot. Yes I know you have changed the pump with no result but if the late type pump has the "O" ring missing in all of your pumps that might be your problem.
Now this problem has just happened and you say that you have not changed anything prior to the low oil pressure when hot problem rearing it's head, a couple of bad things that could cause this, one has been mentioned worn big ends, or maybe the sludge plug leaking or if later big journal crank the centre sludge trap flywheel bolt that is drilled into the crank's oil chamber loose or leaking. Muski mentioned the outer part of the combo needle roller bearing being loose and not sealing the old oil hole in the in the timing side case might be the inner ring of the bearing loose on the shaft journal and not sealing the old oil hole in the crank. Has the outer timing cover an alloy block welded inside it to hold the oil chamber and seal for the crank is this cracked somewhere or is a weld leaking when hot. I would look very closely at the PRV thread as they have been known to leak between the inner thread into the oil relief chamber then bleed off pressure, SRM's PRV as mentioned has the first part of the thread made oversize to "bed into" the female thread in the cases, the instructions they give say that you must back and forth it into the case and there is a torque mentioned that the PRV should give as it initially screws into the cases to form a tight thread. The leak is somewhere keep looking!
