Not sure what year they revised the valve springs, but a buddy had an 04 that would go back and fourth between running fine, and running rough and backfiring slightly through the exhaust, more of a miss really. All signs pointed to electrical issues. Changed the MAP sensor, spark plugs, fuel injectors on the misfiring cylinder (throwing a code), finally ran a compression test and low and behold that cylinder was down. Changed the valve springs no more problem. Food for thought.