As previously stated, replace the sensor. You cannot really clean it out and it is not a true oil pressure gauge sensor. It is simply an on/off pressure switch. If the oil pressure goes above 8 PSI the oil gauge shows normal pressure (always the same spot on the gauge). If the oil pressure drops below 8 PSI the gauge shows 0 pressure. So in reality your oil pressure gauge is just a glorified idiot light.
Sport Tracs are known to experience this problem with the oil pressure sending unit when the temperature starts to get cold overnight and starts to dip into the 30's. After about a minute or two of running, the gauge will flip back on to a normal reading.
It happened to my new 2001 Sport Trac and it was only about 1 month old, so it's not an issue of the sensor getting dirty, just a poorly designed one.
...Rich