First off buy an hand held oil pressure gage. Never tear into an engine bases on a mass produced dash gage. A good gage you can check with your regulated shop air line.

Normally when you get a rapid decrease in pressure when you are not running your car hard and you have no new noise it means your oil pump has failed (like was said above). It could be as simple as a broken regulator spring inside the pump. If you still have the original oil filter you could cut it apart and see if there is metal in it. That would be a sure sign of a major oil pump failure. Again that is most of the time. There are many other things that can case this issue.