your car sounds like the same problem my mum had with her xsara, it just lost power whilst driving it back from a trip to scotland, it got so bad that eventually i just refused to drive it any further and brought the RAC out to it. I had wondered why the previous owner had only had it 8 months, the RAC took it to the dealers it was purchased from and gave us a lift home. It had a faulty ECU and once it came back it took off like a rocket, even if the car is no longer under warranty if they havent sorted it out while it was then they have a responsibilty to sort it out, if its on finance contact your finance company for advice or contact trading standards.