I have a 2001 Volvo S80 2.9, check engine light came on and now the engine idle goes from 1000 rpm to 500 rpm then back to 1000, constantly. While driving it sometimes revs up while coming to a stop. I know this car does not have turbo.
So call them to see what they can do? For me, at minimum it sounds like you may need the PCV system and throttle body cleaned and worse case scenario you will need a new throttle body? Good luck to you and hope this helps you.