How can I replace my fuel pump?

My fiat will drive perfectly and all of a sudden, stop, as if I had braked. Then carry on for another few seconds and do it again. Its strange. I have already taken it to two garages and they cant figure it out. The said it was the coils, so I had them replaced and still had no luck. My dad, a mechanic said it is the petrol pump... its a punto ELX 75 and its a 1996. Head gasket had been done as well as an oil change and new spark plugs and leads. This problem is just getting worse. Im afraid it may cause an accident.