Why would a 1993 Pontiac Sunbird 31 lose oil pressure when idling at proper running temp and then it returns at higher RPMs?

well, oil pumps are power by the engine, when the engine is at idle its at its lowest rpm when you step on the gas the rpms get higher therefore your oil pump works faster delivering oil at a faster rate to keep up. oil pumps got a valve call the pressure relief valve and that valve maintains proper oil pressure at all times. if the valve fails you may experience those kinds of problems. it could also be that the rotors or gears on your pump are worn out beyon manufactures specs. and they not providing proper pressure. another answer to your problem would be low oil level and or faulty sensor.