Oil Pressure light comes on when I first start the truck.

I have a 2001 Ford F-150 with 94,000 miles. The truck has the 4.7 V8. The oil has been changed every 3 to 4 thousand miles since I bought the truck new. When I start the vehicle in the morning the oil pressure gauge will drop to 0 after about 20 seconds and stay there for about 10 to 20 seconds. It will do the 3 to 4 times and then be ok for the rest of the day regardless how many times I might start it. When the gauge is working it is just above the center mark. Do you know what might cause this?