Since you have done the obvious, it is possible that the oil pump has gone bad from wear. Not knowing or mileage on the engine, most everything is a guess. The oil pumps do not wear out very often, but can, and there are stories of bad replacement pumps out of the box. There is also a possibility that there is an internal leak somewhere. Check your turbo oil pickup and return lines, too.
This is not an easy thing to fix or diagnose, but since you changed oil and restored pressure for a little while, my money is on the pump. They can be fixed without being replaced, but you need access to a machine shop. The bottom plate wears from the pump gears and creates a bypass. That can be taken up by grinding of machining the plate to be smooth again.
Crap! I forgot to mention the other possible, and highly likely culprit. The bypass spring in the rear oil cooler head sometimes wears out and does this. It is not serviceable according to Ford, but Icanfixall has changed the spring in his. Perhaps he will check in here.
