[Oaksail]

I'm assuming your gauge is of the electrical type and not a direct pressure type.
Remove the sender and thread in a fitting, flexible hose and gauge and assess the pressure, a 0 to 100 psi gauge should work.
I had the same problem last year, turned out to be a sender wire grounding out at the back of the engine. 0 psi and the sender grounds itself to the block. The higher the pressure, the more resistance to being grounded.
Even though your gauge reads OK when cold your engine will vibrate differently when it is warmed up and cause the sender wire to ground out at a different vibration.