Hey mate. Me wifes 33 does the same thing. If you come of throttle to quick under boost, it wants to stall or drop in idle. It's only since i've put a atmosphere type BOV on. The valve stays open a bit longer and upsets the afm. When you are revving the motor and realising it and you hear that "chhhhhh", it's just the pressure realising out the front housing and not out the valve, like it should. Make sure that the vaccum hose to your valve isn't crimped somewhere, hasn't got a hole in it and check to make sure that valve isn't leaking. The standard valves for a gtr a hiden in the inner guard, not sure its pass or drivers, so if you have custom intercooler piping then you wont have a standard valve. Hope this helps mate.