Does your model have a vacuum accumulator or similar? Several cases on xoutpost when diesel gets winded especially say going up a long hill and power fades, it was a leak in the tank that I think somehow controls the turbo boost. I don't work on diesel I don't remember the details but you can maybe find the thread on xo it was probably in the e53 section

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)