My engine started misfiring and CEL came on. The codes were P0300 (random multiple cylinder misfire detected), P0302 (cylinder 2 misfiring), P0304 (cylinder 4 misfiring). Long story short, the problem turned out to be leaking valve cover gasket. There is a small spring at the end of each coil pack that connects the coil pack to the spark plug. Mine were completely covered in gunk and oil. After thoroughly cleaning the spring and the end of the coil pack (pull out the spring to get to it), misfires stopped. Now I "just" have to replace the leaking gasket. Hope this helps somebody.