P0300 Random Multiple Misfire Diagnosis and Fixes for 1999 GMC Jimmy

I recently encountered a persistent P0300 random multiple misfire issue on my 1999 GMC Jimmy. The truck had been running well for over three months, but during a freeway drive, I noticed poor acceleration and the engine began to run roughly—stuttering and sputtering. The temperature gauge climbed steadily, and the 'Service Engine Soon' light started flashing. I pulled over, and by that time, the battery had died. After a jump start, the engine continued to run poorly with rough idle and noticeable misfires. I towed the truck home due to safety concerns. Upon removing the battery, I discovered acid leakage in the battery tray. A new battery was installed, and while the truck started immediately, the misfiring persisted—especially under acceleration, where fuel consumption increased significantly. At an Advanced Auto store, I retrieved a diagnostic code: P0300 — Random/Multiple Cylinder Misfire Detected. The next day, a local mechanic performed a scan and recommended a tune-up. My friend, who is a certified mechanic, replaced all spark plugs, wires, distributor cap, and rotor. While this improved the symptoms slightly, misfiring still occurred intermittently. He then suggested replacing the ignition coils, which I did—no improvement in performance. However, fuel economy improved noticeably. On a 100-mile round trip, the truck struggled to exceed 70 mph but ran smoothly once past that speed. My mother connected me with a mechanic who worked at a dealership. He inspected the vehicle and found that the distributor cap was not seated properly—its screws were loose. After tightening them securely and verifying proper wire connections to each cylinder, the truck runs significantly better. Misfiring is now random: it occurs once or twice during cold starts but disappears after warm-up. The idle remains slightly rough, but no overheating or over-revving has occurred. I also cleaned the EGR valve, which was not stuck open—performance remained consistent with or without the valve installed. Despite these improvements, I remain concerned about underlying issues: the fuel pump makes a loud noise upon startup, and exhaust sputters. My mechanic suspects a faulty fuel pressure regulator. While some online forums suggest checking oxygen sensors, I’m unsure whether this is relevant given that no O2 sensor codes are present. The truck now runs safely without overheating or coolant leaks, and the radiator maintains proper pressure. I’ve also used fuel injector cleaner and dry gas treatments with limited results. I’d appreciate any insights on next steps—especially regarding fuel pressure regulation and whether a full diagnostic scan (including fuel trims and freeze frame data) is necessary to confirm root causes.