I had similar problem on my Volvo 2001. The engine will misfire and the check engine light will come on. The dealer changed the sparkplugs 4 times, and I finally got tired of taking the car there. I called Volvo, and complained over the phone - that didn't work. Then I sent an official request to Volvo to take care of the issue. To cut the long story short, Volvo sent someone to do a full diagnostic of the car and they found that one of the cylinders wasn't compressing properly (I believe the compression was supposed to be in the range of 170 - 172 and mine was 160) and was the cause of the misfire and the check engine light. The dealer replaced the head? of the engine and the cylinder (I am not a car person, so apologies if my explanation is not that accurate.) That was 2 months ago and the problem hasn't come back. It is my understanding that this is not a problem that should happen at all, not at 40000 miles at least. Let me know if you need any more info on that, will be happy to help.