turbo stealth

hi all can you please help me cure this problem,when i leave the car for a week or two then drive ,if you put your foot down when its not up to temprature it missfires biut gets better as the car gets warmer then after 10 or 15 mins later its gone and runs mint.
Any one no what it could be,i use ngk plugs and everything else is genuine
ford .

regards
rob