Power Failure /Injector pump

[Dolly12]

Hi all,
I hope you can help me out.
The yellow engine light came on as i was driving my Mini One 2007 1.4 petrol and the car lost power as i was picking up speed. this seem to be ok when i stopped and restarted the car. A few days later the same problem. I took it to the car garage Mini garage, they say that Injector pump 4 was misfiring AND THEY CAHNGED THE PUMPS AROUND BUT COULDNT SEE WHY IT WAS MISFIRING. Now i dont much about cars they tell me it is not the convertor or coil pack but that they do not know how to fix the car and have contacted BMW Techinal service. As it stands they are charging me for all the labour but no new parts have gone into the car and its already cost mr 500 + Euro.
Does anyone know about this problem. What i keep hearin from them is that they can not get it misfire but the fault is showing up as injector pump 4 on the computer.
Would be grateful if anyone can give me advice.