Drop in Oil Pressure as Engine Warms up

I have a 2000 Dodge Ram 1500 with 5.2L V8
engine. The truck sounds greaf, but after it warms
up for a bit, the oil pressure drops a bit. When it is
first turned on, the pressure reads great on the
gauge on the dash. After it begins to warm up, the
pressure drops a bit, but still in the good range. But
within 5-10 minutes after it has been idling, it drops
to zero when I get the engine to 3000 rpms and
goes back to in the “good range” at idle. After doing
that a few times, then the gauge just stays at zero.
Any thoughts on what this may be? Is the gauge
faulty? Or is there a bigger problem?