focus 53 plate problem

[Punt99]

Hi Guys, need some advice if possible to a recent problem

I have a 53 plate focus and its developed a problem which is making it un-driveable.

you can be driving along then all of a sudden the power is gone, you can have your foot hard on the floor and nothing!, sometimes if you take you foot off then back again it springs into life but generally it will judder along losing power, if I gear down it will be fine are rev right up, some days it will be completely fine. 

I don't think its clutch slip as the revs don't rise when no power etc and I've changed the plugs and leads. My next fault finding step is changing the coil pack but im starting to think a fuelling issue??

I am a very novice mechanic so any suggestions would be grateful!!

thanks