This post is for my 2004 Discovery G4 w/ 60,000 miles. Recently got the Service Engine Light. Checked codes: P0300, P0301, and P0306. Understand that these are for cylinder misfires. I replaced plugs w/ Bosch Platinum +4's and reset the light and ran fine. After about 120 miles of highway driving, Service Engine light came back on. What's next, new plug wires? Any help steering me in the right direction would be greatly appreciated.