Ford Focus TDCI - Focus losing power - Dieselian

I have a Focus TDCI 2002.It has been a bad starter since i bought it a few weeks ago and then occasionally it loses power.I have to pull over and switch off then on and it can be fine for the rest of the journey.A garage told me told change the sensor switch on the fuel inlet manifold.I did this at the weekend and now the car is off two cylinders.