On my s60 2.0 t I had very erratic acceleration and idling. To explain the symptoms: the car idled fine and suddenly dropped idle wanted to stall and then picked it up to 1000rpm and then back to normal. There was no pattern no exact time between these problems. Acceleration was slow at first and then got better once the turbo kicked in but was not perfect. After checking through MAF, Throttle body, boost sensor, intake air temperature sensor, cleaned turbo recirculation valve and checked for boost leaks... I decided to replace all vacuum hoses for the turbo and others too. Those are very easy to do. During the work I FOUND one of my NON RETURN VALVES (I have 2 of those) were letting the air through both directions. Could not tell just by looking at it but once I blow air through it from both ways it was obvious.
Put it all back together and voila
. Hope it helps