The best plugs for a zetec are genuine Ford ones, they do often run a bit funny with different make ones. Gen. ford are often cheaper that other ones too, think i paid £3ish quid each for mine.

Misfiring is more than likely down to a knackered coil pack, they do fail commonly. Ford do sell genuine ones and pattern part ones. I got one from a scrappy for my girlfriend's fiesta. Just take your chances really, as long as the part number on the sticker on the top is the same, there is only 2 or 3 different types across the ford range for the zetecs.