Based on your symptoms and all the things you have already checked,
Sounds like you could have a low boost situation.
Do you have a boost gauge, if not, adding one can tell you volumes as to whether the problem is lack of air/boost related.
One of the first things to check is the stroke of the turbo vane actuator to make sure the turbo vanes are not sticking or hanging up and not allowing full boost when called upon.
Another thing that can cause low boost issues is a cracked or partly collapsed vacuum impulse line between the turbo vane actuator and the turbo modulator valve.
Several including myself have found this little plastic tubing damaged or partly collapsed and have replaced the plastic tubing with high quality silicone hose.
Of course a leaking CAC (intercooler) or EGR valve leaking can cause loss of boost if you still have an EGR system in place.
The FCV can also fail and stick partly closed limiting boost to the engine.
Most CRD owners remove all the EGR, EGR cooler, and FCV crap off this engine since they are nothing but engine killers.
A stopped up CAT can also limit turbo performance and cause low boost issues due to high backpressure on the exhaust system.
The CAT can be removed with NO adverse affects, only benefits, many owners have done this.
