Engine running very lean?

The car has been running very lean at idle. Short term fuel trim goes up to 30%+ and long term 25% or so. As soon as I press the gas pedal and start driving, long term drops down to 18 and short term also comes down rapidly. While driving at cruise speeds short term is 5 to 6% and goes negative from time to time and long term fluctuates between 11 and 14%

There are no drivability issues except when idling for more than a minute then it starts stumbling somewhat but doesn't shut off.

ignition coil, spark plugs, wires, EGR valve have been replaced. Did a smoke test and didn't find any vacuum leaks. Also checked for exhaust leaks by spraying soapy water and pushing air through tail pipe and looking for bubbles. I didn't see any so now I am checking the fuel system.

It's a Zetec engine with return-less fuel system. The fuel line connects to the pressure regulator and pressure regulator is bolted on the side of the rail. There is no Schrader valve anywhere or any hose between pressure regular and rail to install a T and connect fuel pressure gauge to check rail pressure. The scan tool shows rail pressure to be 39psi, which is what the manual says it should be.

I connected fuel pressure gauge to the fuel line just before the pressure regulator and the gauge reads 30 psi. Is scan tool showing the wrong reading or pressure regulator is increasing the pressure from 30 to 39. Is this how it should be? If so, then what else could it be?