plugs/wires come to mind, first, as long as idle rpm is right around normal. (not drastically below normal)
Duratec plugs should be Autolite APP764 Double Platinums, gapped to 0.054.. wires are OEM Motorcraft replacements or the expensive ones which are Magnecors (better performance, but much more expensive)
check the gap, and condition of all plugs, and make sure the plug wires are still connected securely and are clean.
Avoid mixing up the wiring order, as the coilpack and cylinder wiring order are different for two plugs (6 and 2, I think)
for Zetec, ignore the wiring order thing I said, as that is a different ballgame.
Ray
