Ford Fiesta 2000 Zetec s 1.6 Mis Fire?!?!?!?!?

i have a zetec s and its mis firing on harsh acceleration and has a slight idle misfire i have replaced the coil pack twice and the spark plugs but i have little money and because its close to chritmas i dont want it to cost an arm and a let but i have been reading up and i was wondering whether it was the air flow meter? there does seem to be a contant hissing from the air filter (K&N) its fine when accelerating slowly to 4000 rpm but if i boot it the cars says no and splutters any ideads?

chris
November 2009