Check your intake filter boxes. I had a similar problem and found a plastic bag had gotten sucked into the intake and was blocking one filter, drivers side if I remember correctly.
If the problem is speed related, which it sounds like, then it could be a problem with the drive line (drive shaft, rear end, axle shaft). If it is RPM related it will likely be the engine.
Good luck!
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RJM pedal, Hotchkis Sway Bars, FI TDX, FOG Light, Z1 Slotted Drilled rotors, Stillen SC, Quaife ATB, Z1 Oil cooler, AEM Gauge Set, Galeforce gauge mount, Avant Garde M590 Hypersilver w/RE-11, TopgunZ A2A kit., ECUtek tuned by Seb: ROLL TIDE!
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