there was a mention before about Inspecting the two rubber intercooler hoses. Also check for any sort of vaccum leakage from any intake pipe/hose leading into turbo (or away on the exhaust side). If you haven't already done so check your EGR valve. As it is outside the scope of a normal service, they rarely are cleaned unless something goes wrong. If it is caked up that will also affect intake into the turbo. Do not under any circumstances accept "replace the turbo" as a first response unless it is obvious. 1 percent of actual turbos fail due to manufacture defaults. Check hoses, pipes, oil pressure, intercooler.
