Have you tried using the onboard diagnostic system and pulling codes? Also, did this problem happen when you first bought the car, or is it a recent occurrence?
That's important to mention all the lights and codes that are appearing. Like most modern vehicles, your Z can respond to any system failure in the car. This could trigger a reduction of revs for safety reasons. If your Z is anything like our USDM Z33s, you'll find the ABS sensor harness is located near the rear diff and branching off to both axles.Yes the car runs fine under 4000 rpm and I just forgot to mention I had the ABS Light on beacuse I have code C1106 rear left abs sensor fail. I will check if it’s the Abs sensor.
I just bought a new set of spark plugs and the rear abs sensor. I just did some tests on my actual abs sensor and it seems to be wrong. So I'm waiting to receive my purchase to change it and try again if the engine runs above 4000 rpm.Beyond the ABS light, have you tried a backup key? If your Z is anything like our USDM Z33s, Nissan provided valet keys that restricted speed and revs for owners. My factory valet key has a burgundy-colored handle.