2008 Mazda 6 2.0 Diesel P0299 OBD2 Code: Turbo Under Boosting - Diagnosis and Solutions

I own a 2008 Mazda 6 2.0-liter diesel model. Before Christmas, the engine management light (EML) came on. I connected an OBD2 scanner and it indicated a lambda sensor fault in the exhaust system. I replaced the lambda sensor, and the warning light disappeared for several weeks. However, yesterday the EML returned. After re-scanning, the code P0299 appeared: 'Turbocharger under boosting'. The vehicle has been experiencing loss of power and sluggish performance over the past few days. While driving, it seems to regain power after a few minutes. I'm wondering if this indicates a failing turbocharger or a blockage in the turbo system. Any advice on whether a new turbo is needed or if there's another potential cause would be greatly appreciated.