Ford Fiesta Zetec Chugging?

Hi, I was wondering if someone could please shed some knowledge on a problem with my wifes car, she has a ford fiesta zetec 1.4 petrol car 04 model
lately it has developed a chugging motion in 2nd and 3rd gear, and is becoming very annoying and getting worse by the day.
I tried some redex in the fuel to see if thsi would achive anything but no luck, just wondered if someone could point us in the right direction,

P.S no illuminated warning signs on the dash