Shuttlefever,
What viscosity oil are you running?
Oil pumps are known as a constant displacement pump. That is, for each revolution of the pump, it wants to push out the same number of cc's of oil. This type of pump will generate whatever pressure needed to push out the design volume. What limits the pressure is leakage around the seals and close clearances in the pump body. Pressure continues to build until the various paths for oil flow accept the requisite volume. This is why these pumps come with pressure relief valves.
So what determines the pressure required? Size of the oil galleys, clearance of the bearings, viscosity of the oil, temperature that modifies the oil's viscosity, pressure relief valves, and leak paths. The pump is content to generate whatever pressure required to deliver its design volume, even 0 psi.
Your last post expresses the concern that the pressure relief valve may be sticking open, however you describe exactly what would happen when the oil thins from rising temperatures.
At normal loads oil temperature is near the coolant temperature of 180 degrees. SAE 30 weight oil at 180 degrees is 14 centipoise, SAE 40 measure about 21 centipoise. As you exercise your engine, cylinder blocks, heads, and oil temperatures rise and the oil thins. At 240 degrees, SAE 30 thins to 6 Centipoise and SAE 40 thins to about 8.5 Centipoise. As the Centipoise drops, the resistance to oil flow through the various passages drops and thus the required oil pressure drops.
If you pull over and let the engine idle, it will take quite a while to dissipate the heat stored in the engine block, heads, oil, etc. As the engine cools, oil pressure slowly builds.
I suggest you get a non contact thermometer from Harbor Freight and watch to see if oil pressure recovery matches the drop in the engine block temperature.