Volvo's are a pain to diagnose, But i would check several things, first is fuel preasure, make sure ya have proper preasure at the injectors /or fuel rail, most old Volvos had 2 fuel pumps,one in the tank called a supply pump,and one on the frame rail called the main pump, have seen more than a few misdiagnosed
because of the supply pump ( not all are aware they have 2 pumps), also check all the air intake boots & clamps to the throttle body,turbo, & intercooler, check all the engine vacume hoses and make sure something hasn't come disconnected, maybe a wastegate problem causing overboost??? hope some of this helps....