Having bought a rover v8 engine for a cobra project I stripped it found bores alright so reground crankshaft fitting all new bearing shells. From start up oil pressure is 30lbs at tickover. As engine warms up to temperature oil pressure deminishes to zero & tappets start rattling. What is the problem
If the pressure is always high again when you restart the engine cold, there could be excessive bearing clearance or the oil is too thin. Also consider plugged drain back holes from the heads and oil is hanging up there. In a fresh engine that would typically be caused by using the wrong head gaskets or installing them reversed front-to-back. Look in the oil fill cap right after you stop the engine to see if there's lots of oil there. You would also find that by the pressure being okay after just stopping the engine for five or ten minutes.
If the pressure goes low faster and faster each time you start a cold engine, consider debris in the oil pan that is collecting around the pickup screen. Usually that won't float away when the engine is stopped so the pressure will be low sooner each time you start the engine.
If the oil level is too high, it will get whipped around by the crankshaft and become aerated. That will lead to low pressure because air compresses.
I know this is rather involved but we found the cause of low pressure once on a fine running engine by using an old brake fluid bleeder ball to pressurize the system through the sending unit port. You might be able to fashion a similar tank from a water well tank. I'd consider that a last resort if you can't find the cause any other way.