Volvo: Volvo S70 GLT Turbo problem, volvo s70, maintenence

Question
My son's '98 S70 GLT is having power loss at idle until you depress accellerator. Also has experienced loss of power while driving. There seems to be some sort of turbocharger or turbo control valve problem.  Any ideas?
Thanks

Answer
i know how turbos work but i'm not that familiar with fixing them; does the owners manual say anything about maintenence for turbo? since they use exhaust gas pressure for a power source (forcing intake air into the engine) i'm sure the components get sooty and dirty. i would think there is a computer code for a turbo problem but not sure (check engine light comes on)