Fiat 500 loss of power in higher gears

I am having a terrible time with my Fiat 500 1.3 Multijet Diesel. Every time I go into 4th and 5th gears the car suddenly slows down and loses power and even when I put my foot down on the gas it will not drive over 40mph. It has been in the garage 3 times since I have bought it (Sept 2008). The first time the warning light came on saying the particulate filter was clogged, so this was then cleared. The second time they said the air flow filter needed to be replaced, but then a week later I had the same problem. The third time I brough the car in they told me that I had contaminated fuel(!) and needed to change the fuel filter. This was apparently fixed, yet two days later I have EXACTLY THE SAME PROBLEM, with loss of power, the car is shuddering in higher gears and it is really a scary drive. I did ring the two main petrol stations where I fill up with Diesel and no-one has reported any contamination issues in the past year at either. Fiat customer services have been appalling and I am left with a car that is only 11 months old that I am scared to drive! Any solutions?