SilverBullet

So I've had a look on here but can't spot the exact query. Our Legacy starts fine every time which is great. But then when it's under load it starts missing between 2 to 3k, over three it then seems to be fine, which is good. So far I've cleaned the O2 sensor swapped the plugs, but no joy. So I'm left thinking this is the a/f sensor or the knock sensor which might need replacing? Any thoughts? SB