Some Zetecers have had a simillar problem. Turned out to be plug wires every time.
The rear bank of the Duratec has some sharp edges on the regular V-6 UIM that can nick or cut the wire insulation. I don't know if the SVT UIM will have this tendency or not, but it may be worth a look. Be sure that braided ground wire to the corner of the coil pack is in place real good too.
Try swapping the #3 plug wire for one of the others to see if the problem follows one of the wires.
