What would cause a Chevy 350 engine to have oil pressure when cold and lose oil pressure when at running temp?

When the engine is cold, all the parts are tight. Meaning, the bearings, rods, and all have less gap. The oil is also thick. As the engine warms, the oil also warms making it lighter or more fluid. If the main bearings have too much gap, then with the warmer oil the oil flow will increase which in turn will lower the oil pressure. It would not be abnormal to start out with 50 or 60 lbs then have it reduced to 20 lbs at idle. Although the pressure would increase to around the 50 lbs. mark as the engine rev's up. If the oil pressure goes down to say 10 lbs, even at high rev's, then have the pressure checked with a mechanical gauge to confirm the reading and if so, well, time for a rebuild. Low oil pressure will not reach the outer parts of the engine and will eventually cause a lot of damage.