what else could the problem be?

i have a zafira 02 plate which missfires in cylinder 2 yet i have had egr valve fitted breather hoses cleaned new ecu yet still missfires.
majorly it generally starts when i am going up an hill or really putting my foot down,it starts spluttering and looses a little power,which also sounds as if exhaust is blowing yet nothing wrong with exhaust.

mark holdsworth
July 2008