When you say it warms up and drops to 25-30 psi that's at idle I hope? What is it doing at cruise and WFO?
A good first step is to connect a good mechanical gauge and re-test.
If you suspect the relief valve is sticking open, it's not hard to take it out and inspect/clean the spring and poppet. See if there's any crusty-ness in the passage where the poppet rides. You can do this without removing the pump.
That said....I don't know if a sticking valve would render close to ZERO psi?
