The only thing more notorious on a 6.0L Powerstroke than head gasket and oil cooler failures surely has to be a sticking unison ring on the turbocharger. A 6.0L Powerstroke features a unique variable geometry turbocharger. The PCM uses an electronic solenoid to allow oil pressure to force a valve to move. The valve is connected to a unison ring, which is connected to all the vanes. The vanes can be opened or closed to allow the engine to build and maintain proper boost levels.
The main failure of this turbo is the unison ring. The way that it sits on the center housing makes it a culprit to stick often. Early turbocharger models are more susceptible to getting stuck as Ford did slightly change the design of the center section. The redesigned center section has a beveled edge where the unison ring sits to it, while the older ones are flat. The ring generally sticks around the inner ring as tolerances are tight.
With buildup of carbon and rust, the tight tolerances cause the ring to seize. Depending on where it seizes, you could have several different symptoms. If the ring is stuck closed, you will notice quick low end boost building up but lack of power at cruise and high RPM. If the ring is stuck open, you will have a hard time even reaching high RPM. It could be stuck in the middle or just not be able to make complete full sweeps causing less dramatic lack of response and power. The key way slot in the ring where the valve arm rides can also wear becoming too large. The unison ring could also be cracked, but this often happens when someone tries to pry the ring off the center housing.
Originally Ford required the replacement of the turbocharger to correct the issue. That thought has since happily died. Now Ford recommends cleaning the turbocharger to restore performance. In several cases, the unison ring requires replacement. Ford does not list the part number for the unison ring inside of their official parts catalog, but supplied it in a TSB. The workshop manual has since been updated to include a procedure for restoring a turbocharger.
Another common failure point is the VGT solenoid. If the solenoid fails, you will have similar symptoms as a stuck unison ring. If the failure is electrical there should be DTC codes pointing you in the direction of this solenoid. A mechanical failure will not show any codes pointing directly at this solenoid. The VGT solenoid is easily replaced with the turbo still installed. Other than a blatant electrical failure in the VGT with associated codes, the likely solution to your concerns is still the unison ring.
The first steps in cleaning your Powerstroke turbo is to remove the turbo from the vehicle. This can be a daunting task for most DIYers but I will try to guide you through the process.