1998 GMC Sonoma 4.3L Engine Misfire P0306 - Diagnosis & Solutions

I'm experiencing a persistent misfire on cylinder #6 in my 1998 GMC Sonoma with a 4.3L engine, indicated by fault code P0306. My OBD2 scanner shows the issue, but the truck starts and runs smoothly under normal conditions—only exhibiting a noticeable 'miss' at idle. I've already replaced all major components including spark plugs, plug wires, distributor cap, rotor, coil pack, head gaskets, and both cylinder heads. Compression readings show 125 psi for cylinder #6 (within acceptable range), while the other cylinders read between 130–150 psi. I confirmed that cylinder #1 is at TDC with the distributor rotor pointing directly to terminal #1 on the distributor base, which aligns with factory specifications. However, I've noticed a significant timing discrepancy—my timing marks are off by approximately 10 degrees. All lifters and rockers have been replaced, and camshaft lobes appear evenly aligned across all positions. Despite extensive repairs and investment over time, the misfire persists. My scanner does not display fuel trims or freeze frame data for P0306, which limits my ability to diagnose further. I'm seeking expert advice on potential root causes—especially given that this engine was originally installed in a 1990 model year Sonoma (not equipped with a camshaft position sensor), and the current distributor is known to be timing-sensitive. Any insights into possible causes, such as ignition timing misalignment, faulty coil pack, or internal cylinder issues, would be greatly appreciated.