The 6.7 does not have a wastegate. We have a VGT (Variable Geometry Turbo.) Which means, the turbo vanes are actuated depending on the boost pressure needed. In lamens terms, if the boost pressure gets too high, the turbo will actuate to produce less boost.
This sounds like a good idea, but with the DPF going into REGEN and over fueling the engine to get the temp up, the soot builds up on the turbo shaft, and jams up the actuating process. If your exhaust brake starts cutting in and out, then that is what is happening. Mine started doing it, then I drove a full tank highway at high RPMs, and it cleared it up. I haven't had a problem since I did the delete.