P0300 Fault on Cold Start - 1998 Chevrolet Astro Van with 4.3L Engine

I'm seeking advice from anyone who has experienced a P0300 misfire code that only appears during cold starts on their 1998 Chevrolet Astro van equipped with a 4.3L V6 engine. The issue is consistent: the engine runs fine when warm, but shows a misfire (detected by OBD2) when cold and under load. After replacing key ignition components—including coils, spark plugs, distributor cap, rotor, and wires—over 22,000 miles ago, the problem has worsened. The vehicle currently has 144,000 miles on it. It starts smoothly and idles well in cold conditions, but begins to exhibit misfire symptoms under load as temperatures rise. Once fully warmed up, the engine runs normally with no issues. I've checked fuel trims and have observed values between 120–126 during cold operation, with negative single-digit percentages (indicating lean conditions). Despite these readings, the OBD2 trouble code persists only when the engine is cold. I'm wondering if this could be related to a failing oxygen sensor or other cold-start-related system issue. Any insights would be greatly appreciated.