Ideally you should Run NGK platinum plugs though I think that maybe what you've used as the end of the ref is 13 for zetec something like PTRA -13 though can't remember at the mo.
Deffo something wrong if your melting them like that though as I've never seen a set of those melt, I use them all the time on my TB's 2.0 and no probs.
Sounds like as said above a timing issue of some sort perhaps