2005 Ford Fiesta 1.3 Problems

[thespecialk]

I recently bought a 2005, manual, 1.3 duratec fiesta. Everything was fine on test drive. When driving the car away after buying it within the first 2/3 minutes I experienced loss of power. My foot was flat down on the accelerator and the revs dropped all the way down to 0 before the car suddenly leaped forward with RPMS at just over 2000 and the engine was juddering. I turned the car off and on again and it drove fine. I notice that the car does seem to lag as if it is not getting enough power, in general it feels sluggish. I put my foot down and it takes a while to take off.

The fault occurs 4/5 times of being driven and always within the first 3 minutes. If it doesn't happen in 3 minutes then it won't happen at all. If t does happen once the car has been turned on and off again it drives fine and wont happen again for the duration of the drive (excpet the general sluggishness is consistently there). I called the seller (I have a one month warrenty on it) and brought I brought it back. The fault did not occur once when he drove it (so annoying!) but he changed the accelerator and changed a number of the sensors. I picked it up today and it seemed fine (excpet for the sluggishness) but later that evening it happened again twice! Now we need to bring it back again and he said he will switch out the contol unit with another fiesta he has there for parts.

I am not 100% sure that this will solve the problem and so any suggestions / ideas / similar experiences would be very much appreciated!