How can i fix my Ford Cougar 2.5 V6 (1999)?

I am looking for answers to a problem i have with my Ford Cougar V6 2.5ltr 24valve Duratec (manual).When the engine is warmed up after approx 10ish minutes then for some reason the car will not accelerate properly.it feels like there is something holding it back.Also , it feels like the accelerator pedal has more play in it too.When the car is cold or warming up to proper temperature it sounds and feels normal and acceleration is great.I have tried various fuel system/valve cleaner solutions in the tank.Car has full service history and i recently changed the HT leads and spark plugs.I have not noticed any drop in mpg or any other problems.diagnostics computer says there is no faults.Any help or advice(apart from sell it,lol) would be appreciated.Thanks :)