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find a local mech who has a "smoke machine".....a quick smoke test will show any intake/vacuum leaks which is what it sounds like you have......

if this is under warranty i would demand the dealer handle this, or call mini-usa to lodge a complaint, i know bmw will consult with their dealers to solve problematic cars.....and being that mini is owned by bmw i would goto the top, if not you'll be chasing this after the warranty is up.....