Sorry about the suggestions to replace the plugs. I just figured it was a cheap enough thing to just "go ahead and try".

To add a bit of confusion.... at 10 years old and 225K, it's time for the fuel pressure sensor to go bad if it hasn't been replaced yet. When it does start to go out it will cause random missfires across all cylinders. It rears its ugly head under heavy acceleration. Volvo had a problem with these around 2002-2004 (not sure of exact years) and issued a bulletin about it. For certain VIN numbers they will provide one free replacement. Call the Volvo owners help desk to get the real scoop rather than relying on a particular dealer. I've found they don't always respond the same.

We had sensor go bad on both my wife's 05 S80 (turbo) and my 03 V70 (NA) within the course of about six months. On the S80 she would get bad missfires when accelerating up an onramp, etc. but it took for ever to get a fault code and check enging light. On my V70 I got the CEL immediately - go figure. On at least one of the cars, I think the S80, I was able to attach a fuel pressure gauge and detect a fluctuating fuel pressure that coincided with the engine stumbling.

Just food for thought.....