they not that expensive, i would be checking your plug gaps too and ford revised it aint the gap written on the tune up sheet under your bonnet. 1.1 is ringing a bell for some reason either thats what it should be or what it was cant quite remember, i remember changing millions of bloody spark plugs under warranty, the coils are cracking asking too much of the coil to ionise the gap in the plug, also if you have a tune good possibility that plug gap is too big.