My 99 2.0i that I had used to misfire randomly like this, I just fitted a new set of plugs and it went away, mine had done around 10k or so on that set of ngks when it started, and while that's probably way short of the service interval for plugs, in your case they are still three years old, mine always ran slightly smoother in general on genuine Volvo plugs as opposed to ngks too.
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02 V40 T4 S, "Sargent silver" she's back!
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