I had exact same problem recently, only happened about 2-3 times, randomly. Like you, seems to occur in higher gears, 3-4 or 4-5.
Changed spark plugs and seems ok at the moment. Spark plugs were very loose when I changed them.
Also got cam follower change on the 'to do' list.
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Hmmm interesting. Mine has been like this for nearly 2 weeks now and is wrecking the usual smooth ride. Certainly hope it isnt the spark plugs as they replaced those during service a mear 5 months back.
Short of changing the N75 and the PCV plus hooking it up to a VAGcom im running out of ideas