The gauge in your car is electrically operated and is controlled by an oil pressure switch located on the passenger side of the engine adjacent to the oil filter housing: it's about an inch and a half in diameter with one terminal and has a braided oil line coming out of it. If the needle fluctuates with sudden changes between the high and low readings, then it is surely electrical so I'd suspect either the switch (expensive) or the wiring (doubtful, but cheap to fix). The oil pressure relieve valve is located on the driver's side towards the rear of the engine and is nothing more than a plunger and spring. You can increase its relief pressure (causing a modest increase in oil pressure in the engine) by adding a copper washer of the proper diameter under the spring, in effect increasing the spring tension. It is a bear to replace the relief valve as you have to simultaneously push in on the nut with increased spring pressure while trying to catch the threads so I'd only attempt this if you are particularly nimble in tight spaces. My bet is it's the oil pressure switch. Good luck.