1997 Ford Aerostar oil pressure

Over the last few months, I noticed that the dashboard gauge shows a change in oil pressure. At times, especially after a long drive, the pressure will drop and remain low (and stay constant) until the engine cools down and is restarted. Starting the engine again, the pressure will be back to where it always had been. At all times, however, the gauge shows that the pressure is in the "normal" range, but I am concerned that there may be a incipient problem with the oil pump or pressure regulator. Oil level is fine. We have a long trip planned in the near future.