Was this pressure drop verified with a mechanical gauge ? Could be as simple as a bad sender .

Deep, yes it was verified with Mech gauge.

The pressure regulating valve in the oil pump may be hanging open or moving sluggishly . This would explain why shutting the engine off and restarting it see's a return of pressure . Stranger things have happened .

Man sure sounds like a filter problem that I had with a Fram that sucked down to 0 turn it off restart fine go a minute suck back down to zero.Replaced the filter all was fine But that was that old crappy Frams they were selling with the valve in them that would not allow the filter to drain so you did not have a dry start after setting .

Seemed like a good idea at the time but it failed.

Could be the pressure regulator valve in the oil pump or maybe the oil pump has been grooved by something in the oiling system.

^^I think this is the problem^^

What's the pan capacity? If it's a standard five quart pan it's pumping the pan dry. Standard pump it's all good, move to a high volume pump, another story. Had the exact same thing happen in a jet boat years ago.

I will find out the pan capacity.

If your not careful welding the pickup on the oil pump, you can overheat the spring in the pressure valve and cause it to fail to close the valve properly. Is there a 'TRAP' in the pan sump. Maybe door is not working and the sudden acceleration causes starvation. Pick up too far off bottom? Filter adapters often have a bypass in case the filter fails and it's spring loaded (I plug them with a 1/4" plug)

So many "little" things can screw with oil pressure it can drive you nuts.

What is oil temp?