Some early 2.3L Rangers had problems with oil pan gaskets. With constant heating/cooling of the engine, the cork gasket would become hard and start to disentigrate. The pieces of cork would fall in the pan blocking the pickup screen.
Symptoms were: good oil pressure at start up. As the engine warmed, pressure would fluctuate, dropping as engine speed increased, rising again at idle, and variations thereof.
Cure is to replace the pan gasket.
Some have been able to drop the pan enough to get the crud out. The pans will not come off with the engine in the truck.
Some have been able to get enough of it out of the drain plug hole to clear the problem using a coat hanger or some such.
Most pull the engine, clean everything up, and install new gaskets. This is the correct way to solve the problem.
However, as others have suggested, make sure it is not something as simple as a bad pressure sender or gauge.

shady