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