1977 Oldsmobile 442 Low Oil pressure at Idle when hot

Engine Mechanical problem
1977 Oldsmobile 442 V8 Two Wheel Drive Automatic 0

I have a 1976 Olds 350cu in engine. We have completely rebuilt the engine and turbo 350 transmission. It runs fine but when the engine is cold it hold oil pressure @ about 50 psi but when it warms up it drops to 1 or 2 pounds at idle. When you up the RPMs the pressure comes back up to 20 psi. We have replaced the oil pump with a high output pump, new sending unit and gauge and replaced the oil with a 10-30 penzoil to see if the other oil was not qualitity enough. We took it to our mechanic and had him check the pressure with his gauge and it did the same thing. We took the top end off and check to make sure the engine shop replaced the plug to the timing gear galley, we spun the oil pump with a drill and we were getting good oil flow in the lifters, distributor port, and through the cam bearings. We are pretty sure that the main bearings are ok as we were careful checking each with plastic gauge strips and specified torque. We don't see any wear on the cam or lifters after the break in. Do these Old Olds engines run low oil pressure and are we chasing a ghost? Are we missing plugging an oil galley somewhere on this engine?

Thanks for any ideas you may have