Zetec misfire and power loss under load
 

I have Fiesta 1.25 Zetec V Reg and have a problem. The car is running fine but when I have to accelerate up a slope I experience a jerking or misfire/flatspot in the car. This happens in 2nd or 3rd gear. I have put a diagnostic code reader on and there is no misfire. I also disconnected the battery to reset the EMU. I've changed the plugs and the HT leads and its still happening. I'd be grateful for any feedback on this.