Apologies for the vagueness of this question but last week my ford focus was spluttering while running and i was advised to have a diagnostic checkto try and find the problem. The chap who done this advised me it was only running on 3 cylinders and suggested replacing the coil, leads and plugs. This was done and everything seemed to be fine until today when the same thing happened. This time it appeared to be a different cylinder not working. I wondered if this was likely to be a faulty coil or another problem, any advice greatly appreciated.