Injector orings would only cause oil in the fuel, or a no start period. Fuel psi is only going to be affected if the fuel pump is bad, filters are bad, restriction in the supply or a bad fuel psi regulator. Now it is cheaper to replace the pump, but 9 times out of 10 the pump is perfectly fine. There are tests to check the pump itself but you would need to have the special tools for that. In the mean time what you can do is restrict the return line, dont just clamp it off but while it is runing take a pair of hose pliers and start appling psi to the return line. What this does is the same a the regulator we are trying to stop/slow down return in order to build higher fuel psi. Watch your gauge while you are doing this and see , now if it builds your regulator is bad/not working good enough. Also see how it runs with the higher fuel psi I have usually set these to run at 60 psi I have seen them as high as 80psi
All the online diagrams only go back to 98 nothing older than that, Is your check engine light still on , hows the engine power/hows it runing?