Alright so I have a 95 Jimmy with the CPI injection. The past few months its been running rough and weak. After letting it sit for a couple weeks I started it up and it ran like garbage and I had to step on it to get it moving because it sounded like it was missing pretty bad. I assumed it was the fuel system again. I got a fuel pressure gauge and tested it. I relieved the pressure and hooked it up.

Turn key on: jumped to 30psi
running: jumped to 50psi
10mins after turnoff: dropped to 40psi
half hour after turnoff: dropped to 30psi

Relieved pressure to try it again
turn key on: jumped to 10 psi
running: jumped to 30psi

I just replaced the injector almost a year ago and the fuel pump and filter a year and a half ago. What does it sound like the problem is? All ignition (plugs, wires, cap, rotor, distributor, coil) are all under a year old. It is throwing an O2 sensor code, not sure for which one.