im guessing that you cant do the work yourself so whatever the answer here you will need to take it to a garage. So if you go to the garage, tell them the problem, they can diagnose it properly because the car is in front of them and they will also repair it. Unless you were just curious and had no intention of getting it fixed
i have had similar problems and it turned out to be air getting into the injectors. try changing the pipes inbetween the injectors. not a expensive job