What do these symptoms indicate?

1998 Ford Ranger 4 cylinder with a rough idle/misfire

There's no check engine light.

Its got new sparkplugs, ignition coils, new fuel filter.

When you start it up, it runs really rough like it's about to stall out but then after a few seconds it "evens out" but doesn't fully stop running rough.

I hooked up my fuel pressure tester to the Schrader valve and noticed that the fuel pressure is low. My manual says it should be at about 40 psi (35-45 is normal according to chilton manual)

However I'm only getting about 28-30 psi when it's running, and about 25 psi if you just prime it but don't start it up.

I then hooked up my fuel pressure tester directly to the fuel filter which is under the truck coming from the fuel tank, and primed it a few times to test pressure, and I only got 27 psi max,

BUT it drains out quickly, within 4 minutes it had gone down to 15 psi.

Could this be a sign of a bad fuel pump? Or something else?

Oh, lastly, with the truck running, if you disconnect the vacuum line from the fuel pressure regulator, the fuel pressure doesn't increase, it stays at 30psi.

Fuel pressure for a 98 2.5 should be more like 60 psi… Specs say 56-72 psi. It sounds like you have a weak pump but could be other issues like a faulty pressure reg. Pull the vacuum hose from the reg nipple and check for any raw fuel in the reg nipple or end of the hose or if fuel dribbles out of the nipple while engine is running… If so, the reg diaphram is bad… Replace the reg. Any black smoke from tailpipe when starting?

Check for proper power at the fuel pump too… Should be 12+ volts when the circuit is energized during the key-on priming feature or while engine is running. If you have damaged or corroded wiring to the pump, that can lower voltage which in turn causes the pump to put out less flow/ pressure. DC motors/pumps are voltage sensitive so lower volts can affect proper performance of them. If the voltage is much less than 12, then test for voltage drops at the pump relay or inertia safety switch, harness connections, etc…

Check for any vacuum or air intake leaks which might also cause erratic idle… Also check/clean the EGR and IAC and MAF/ throttle body of any carbon/soot buildup… Likely due for cleaning on an older 98 model.

https://www.ranger-forums.com/sohc-2-3l-2-5l-lima-engines-135/low-fuel-pressure-143542/

http://www.therangerstation.com/tech_library/4cylinders.html

Looks like a bad fuel pump. Remove the vacuum hose connected to the fuel pressure regulator. If the hose smells strongly of fuel or the regulator is leaking, the regulator is likely bad too.