Did your mechanic check the lower intercooler boost hose?
Philia Bear is a Volvo trained Technician and he gave the diagnostic information clearly.
The lower boost hose is the most common problem with this model in Australia, it cracks/splits and sounds like boost leak (which it is) and the system freaks out. You need to squeeze the hose.
Then again, as it's a common failure, just replace it as a matter or regular maintenance.
This forum has members who are Volvo trained techs from Europe and USA and locally trained Volvo techs and as a group have worked on Volvo of all era for a long time.
This is why the Ozvolvo is the resource that even folk in other countries use for their issues as well.