I read the symptoms off one of these volvo forums and looked at a schematic of the boost control system. It made perefect sense as the valve/solenoid controls the waste gates which were not maintaining boost correctly due to opening erratically. The part was original (10 years old) and a new one cos...

Sounds like the TCV (turbo control valve) also called BPS (boost pressure solenoid) on the side of the air box. I had the same problem and a new TCV fixed it. Got one from FCP Groton or try IPD USA.

Late reply but it may help others......It is the TCV - Turbo control valve. Also know as the BPS - Boost pressure solenoid. It is attached to the side of the airbox. I had the same thing. Surging or steps of power under hard acceleration. A new TCV solved it straight away.