My first guess is still, unfortunately, excessively loose or sticking wastegates. Takes about 5 hours and fresh coolant to get both front and rear turbos adjusted, but if they are really shot, only a new pair of turbos will fix it. Time for RB or Vargas upgrade!
You also need to pressure test the intake system to at least 15psi (not just a 0.25psi smoke tester), check all the vacuum lines for abrasion or dry rot, and check the canisters for any cracked nipples.
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RRT
E92 335i/6MT with stuff and things
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