I am not sure that this will help, but from what you are saying it sounds exactly like the issue I had with my S40: engine misfire & check engine light on. The dealer changed the plugs 4-5 times, before I got sick and tired and called Volvo. To cut the long story short, Volvo sent someone to check my car and they found out that one of cylinders was not compressing properly (if it was suppossed to be 170-172, the compression was 160). Volvo replaced the cylinder and a bunch of other stuff (I am not a mechanic, but I have the paperwork, if you are interested in what exactly they did) and the car has been fine ever since. However, I feel that this shouldn't have been an issue with a new car that has only 40 000 miles on it.