My 2002/ 51 plate Focus 1.6 Zetec has covered 135000. It has acquired an occasional misfire which occurs when you are cruising at speed with the throttle slightly open. It does not happen under hard acceleration. My local independant says it is moisture causing tracking on the spark plug leads where they enter the engine and Ford recommend a dose of their silicon spray. Has anyone heard about this? It is not a great problem because it does not happen too much. They are the original plug leads so maybe it is to be expected?