2005 GMC Sierra 1500 Base 4.3L Engine - P0351 Check Engine Light Returns After ICM Replacement

My 2005 GMC Sierra 1500 Base with a 4.3L V6 engine is running, but the check engine light remains on with fault code P0351 indicating a misfire detected in cylinder 1. I replaced the Ignition Control Module (ICM), and after installation, the vehicle started and ran smoothly initially—no misfires were observed. After clearing the code, I restarted the engine, and while it ran fine during operation, there was a noticeable stumble on the first crank cycle before stabilizing on subsequent starts. The P0351 code reappeared when I turned off the engine, even though no stumbles occurred upon starting. My neighbor, 'The GUY,' previously installed a crankshaft position sensor, which resulted in no start—this led us to try the ICM instead. The vehicle now runs better after the ICM replacement but still exhibits intermittent issues on startup. I'm not ready to replace parts blindly and would appreciate any insights or troubleshooting steps for this specific P0351 issue on a 2005 GMC Sierra 1500 base model with a 4.3L engine.