The coil pack has always caused problems, it's not just Ford either Renault suffer same problem amongst others probably.

If one gets such a problem just a replacement is suffice, for you to have 5 there is obviously something more wrong and I would question why if garages are fitting they are just replacing, I guess it's not under warranty? Do you have any other electrical problems as I'm thinking perhaps it's getting to much power on the input side, PCM problem perhaps?

Martin