warranty on a new car

Hi, I bought a new Volvo S40 end of April 07.
Beginning of september the car became impossible to drive, acceleration cutting obliged to restart and thenn it does the same after 20m.
Since 3 weeks the car is at the dealer and they haven't solved the problem yet.
What are my options at this stage would I be entitled for a replacement or a refund?

Thanks for your help