low oil pressure

Hi everybody, I need some help here… I just bought a 1998 Ford E150 Econoline and I have a problem with the oil pressure. After I drive the van for 10-20 minutes, when I stay at a stop light the oil pressure drops all the way and the oil light is flashing, but if I start moving and I accelerate the pressure will become normal.(or if I put the car in the parking or neutral gear, and the engine is on ). I changed the oil and filter using the one the book recommends (5W30), (the oil level is normal). I don’t understand what’s happened? The engine runs very well and I don’t have others problems. I hope somebody can give me some ideas. Thank you.

You should really post this in the ford area, but I would say that maybe the pressure sensor, or the pump is bad.