Why would 1993 Chevy 350 oil pressure gauge suddenly jump to full scale when engine RPM reaches mid range or above Pressure reading returns to normal after engine RPM falls below mid range to idle?

You most likely have too much oil in the engine OR your oil pressure regulator is faulty OR there is a blockage in the drain galleries or oil filter. I would change the oil (filter as well) and add the recommended amount of oil (5qts i believe but double check for your car) and see if the problem is solved. If not you may need to tear down on the engine, clean it well, add new seals and gaskets, and put it back together.