i have a 2002 (52 plate) 1.4i zetec fiesta and when I excelerate it starts to chug and isnt running properly the engine maintenance light comes on but it start's up fine
My wife's fiesta had an identical problem to yours - I traced it by process of elimination to the Coil Pack - the
" box " where the spark plug leads come from. Changed it and solved the problem.
Apparently it is a common enough problem. They cost about �60 - �70 and are easy enough to replace if you are such a way inclined - just watch if you change it you don't mix the leads up.