P0131 Fault Diagnosis on 1995 GMC Jimmy 4.3L V6 with Intermittent Miss-Fire and Power Loss

Hi everyone, this is my first post here — I'm eager to improve my vehicle diagnostics skills through community input. I own a 1995 GMC Jimmy with a 4.3L V6 engine (W-code), 4x4 and automatic transmission, currently exceeding 200,000 miles. When I first purchased the truck, it had recurring EGR-related issues. I resolved this by removing the intake manifold to clear a clogged EGR passage and replaced the EGR solenoid. After that, the engine ran smoothly. I also replaced the distributor cap and rotor at the time. The spark plugs are NGK, wires appear in good condition, and a new catalytic converter was installed initially — it passed emission tests without issue. Several months ago, I began experiencing intermittent power loss when the engine is hot, accompanied by severe misfires. The truck runs perfectly during cold starts and under light loads. However, once warmed up, it performs well for about an hour before issues begin. A key incident occurred on a highway hill at around 60 mph: I accelerated hard to pass a slower vehicle. The RPM increased, but there was no acceleration — only heavy misfiring. After releasing the throttle, the engine ran normally under light load. No Check Engine Light (CEL) illuminated during this event. Later that day, I managed to accelerate to 70 mph at Wide Open Throttle (WOT) without any misfire. However, the next day after driving for about 40 minutes on the freeway, I encountered a long incline and accelerated from 50 mph. Within 10 seconds, severe engine misfiring occurred, followed by the activation of the CEL. The engine now exhibits significantly reduced power even at low speeds — it only runs smoothly under light throttle or minimal load. My OBD2 scanner shows oxygen sensor readings between 0.1 and 0.9 volts with a strong response when propane is introduced or an air leak is created. I replaced the oxygen sensor due to its age (over 200,000 miles), and it was clean with a light tan color. During one severe misfire event on the hill, my tachometer showed erratic behavior — readings would twitch and freeze momentarily. I recently replaced the ignition module, suspecting an electrical fault. The catalytic converter is silent (no rattling) and runs smoothly when operating normally; it completes all gear shifts above legal speed without misfiring. Engine vacuum remains steady at 19 inches, dropping sharply under WOT — indicating no significant restriction. Fuel pressure has been confirmed at 60 psi, though I haven’t measured it during misfire events. I’ve verified that mechanical components such as valve springs and ignition systems are functioning properly. Plug wires are recent and visually intact. I've sprayed them with water at night — no sparks or stumbles observed. My scanner data (non-professional level) shows stable TPS readings, accurate coolant temperature, short-term fuel trim between -2 to +3 at idle and -6 to 0 at 3,000 rpm. Long-Term Fuel Trim (LTFT) is typically around 0.1 but occasionally spikes to -2 before returning. During a prior EGR repair, I powered up the fuel pump to test for leaks at the injector spider — no leaks were detected. I’ve researched online and found multiple reports of 1995 GMC Jimmy owners experiencing P0131 with similar symptoms: intermittent misfires under high RPM WOT conditions. Some have replaced the O2 sensor, but none have reported a full resolution. I recently replaced the ignition module — I haven’t yet driven it long enough to confirm if this resolved the issue. The main challenge is that the fault only manifests intermittently and requires a scanner during misfire events. Any insights or suggestions would be greatly appreciated.