I too had the same experiance after changing plugs on one. Had it back in to try another set,not a 5 min job,checked everything ,could not find any other fault ,but customer kept on complaining of slight missfire. Air filter was very dirty when we changed it at the service ,i wondered maybe if it takes a long time to adjust ltft or mucky filter could have been masking a previous fault,will never know as customer got a bit snotty about it.
GEORGE