1999 Ford F-150 low oil pressure

1999 Ford F-150 mileage: 125,000. Late last year, my oil pressure dropped to the low end of normal. I flushed the engine, which corrected the problem (at the time). Now, less than a year later, my oil pressure is back down to the low end of the "normal" area of the pressure gauge again. What are some possible causes for this? My oil pump? If so, is there anything special that I need to know about changing my oil pump (i.E. Torque specifications, etc.)?