I had the same problem on my car.
Checked the gauges, tried different oil filters and oils and even drove around with a mechanical gauge sticking out of the shaker hole in the bonnet so that I could see the PSI reading.
It ended up being a stuffed Oil pump.
I replaced the pump with the engine still in the car............on a hoist.
It was easy enough to do. Engine mounts were undone and a screw pole was placed under the transmission housing and the whole assembly was lifted slightly to ensure clearance for the sump to slide out.
I replaced the old Hi Volume pump with a new Standard Volume unit and a Hi Energy anti cavitation pick up. This new pick up is smaller so the Hi Energy Sump I have slid on a lot easier.
I haven't had any issues since.........................but bloody hell did it take a while to diagnose the problem..........