It sounds to me like your HPFP is dying. I had mine fail at around 60,000km. It first started happening when I was overtaking or accelerating with WOT. After driving a bit or turning the car on/off it would be fine for a while. Eventually it started happening more frequently and with even less throttle. Took it into the dealership and they replaced it. Extremely common problem with the earlier N54s.
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