Let me guess; it's only happening when the engine is warm and it's idling. If that's right, the engine bearings are getting a little older and the gap between the bearing and the crankshaft is increasing. It's normal and as long as the oil pressure comes back up when the engine is running at normal operating speed, everything is still ok. If you want to have the oil pressure back in the normal operation range, try this. Next time you change the oil, go to a slightly thicker grade oil. If you've been using 5W15, change to a 10W30. Those numbers identify the "viscosity" of the oil at specific temperatures. The higher numbers indicate a thicker viscosity. If your bearings are worn, a thicker oil will not leak out as fast and will keep the oil pressure up. It will also help the bearings last longer now that they're a little worn.