Next steps for diagnosing a Honda Civic misfire after ignition components, P0304

[Robert_S]

I'm a weekend mechanic trying to diagnose a persistent intermittent engine misfire on my 2008 Honda Civic; the check engine light flashes under acceleration and there's a noticeable shudder. I've already replaced the spark plugs and ignition coils on the suspect cylinder, which didn't fix it, and my basic OBD2 scanner only shows a generic P0304 code. For more experienced DIYers or mechanics, what's the next logical step in engine misfire diagnosis when basic ignition components are ruled out? Should I be looking at fuel injector performance, a possible vacuum leak affecting that cylinder, or a deeper issue like a compression loss or a problem with the variable valve timing system, and what's the most cost-effective way to test these possibilities without just throwing more parts at it?