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Your dealer is correct. I too work at a dealer and see this sort of thing often. Some model year S80s, that's cured by software, sometimes it's cured by a new Mass air flow meter and sometimes it's cured by either cleaning or replacing the throttle unit. There is no one easy answer that I can tell you from here.
We start by looking at Volvos online technical literature to see if the particular year car requires software that MIGHT address the problem. If that doesn't apply or fix it, then it's the next easiest step, assuming that there's nothing obvious like a vacuum leak or codes present. It's easy to try a new MAF meter (yet another common failure item these days on S80s) and if that doesn't do it, the throttle unit is next in line. I've seen both MAF meters and throttle units cause that problem (and others) even when no codes were present.
There are no specs that are important as it's all electronically controlled, no adjustments.