1.8 zetec engine problem help needed?

im new to this so bare with me lol
ive got a mk6 escort gti 1800cc zetec all standard, all of a sudden one day with no warning the car will start fine and run under idle fine untill it warms up or gets hot (the engine fan works) or go for a drive, then it missfires and backfires throughout the rev range and no power at all unless i put my foot down all the way (but still lacks power) then at idle it drops to about 500-200rpm and runs really rough and trys to die, when i try to rev the engine to get it to idle its like there is no power at all untill i use full throttle, ive changed the injectors, coil, spark plugs, throttle pisition sensor, crank sensor, HT leads, MAF sensor changed, cleaned air temp sensor, the head gaskit was done with a head skim about a yr ago, there is fuel in the tank, any help what so ever even if you think it wont help needed
thanking you for your help in advance
joe

joe warrington
February 2010