vnt overboost

[iTurbo]

I've heard that the VNT calibration has a 'soft knee' overboost trigger. I think you have to be over 15 psi for x amount of time before it will shutdown.

[asless]

Does this sound normal for a VNT? 1st and 2nd gears, I get a spike to 16 to 17 psi with no OB shutdown, pulls back to 13 or 14. Third gear same spike but now creeps past 15, still no OB. 4th gear, same spike but creep is above 16 psi, after about 2 seconds of this the car shudders, no harsh kickback of OB like my older T1 did. I have an adjustable zener on, I thought I had it dialed in but apparently not? Do I just keep dialing the zener back? I am reading an OB code after this happens. Does anyone have knowledge of the VNT OB peculiarities?
Thanks
Les