Do this test; get close to the point it will limp mode and coast for a while then WOT and see if you can go a little faster.

If so, look into the vacuum accumulator hardware. Several instances of this being the culprit on diesels. Usually a big enough failure that it happens without towing and just going up hill steep enough. Possibly you have a small leak that only causes a problem at high enough engine load

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