I believe my car has a miss firing problem as well. Its not the AFM because I just stuck a new one in there and also have tried another AFM from another car.

When idles, I see the engine has a real light shudder/shake in intervals. You can sorta pick it out if u listen to it carefully. Plugs were changed at 100k service - How often should these be replaced? Spark plug leads I believe they are factory - Unsure if it has been changed.

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STi Version VI