Because it clears when decellerating, I would suspect a bad vacuum line. Take off the cover on the throttle body and check all of the lines that connect to the vacuum tree. Check the lines that go to the air cleaner and towards the radiator.

If the PCV hasn't been cleaned in a while, take it apart and throw away the plastic screen. Clean the lines that go to the PCV.

The knock sensor will retard spark and cause a rich condition, but that usually only happens at throttle up with regular gasoline and high temperature. Switch to a tank of premium gasoline and then run with at least 89 octane to prevent knock.

If you haven't changed spark plugs in a million miles, pull them and inspect for wear, replace if needed.

I doubt it is your fuel filter, but that is a cheap item to replace and easily done. Book says every 90K miles. New Volvos do not have fuel filters.

Klaus
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Always willing to listen, just not able to take direction.