Mitsubishi Triton 2008: Turbo failure

I would like to know why the turbo in my 2008 diesel Mitsubishi Triton GLXR triton dual-cab just stops working. It happens about once a month, it can be when it's towing or not, the weather could be hot or cold, or when it's working harder. No lights come on, and the turbo does not make any unusual sounds. If I pull over and shut down the motor and re-start, the turbo is back in action. My mechanic hasn't been able to find any faults, and he's changed the air filter and a couple of other parts. What can I do?