K16/16G FVD flat spot on remap

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Hi all,

Looking for some suggestions.I recently got some K16/16G turbos fitted to my 02 Turbo. I have all the supporting mods such as

Uprated Boost hoses
Diverter valves
1 bar wastegates
cargraphic exhaust with 200 cell cats
K16/16G turbos
5 bar FPR
New oil lines

I had FVD put a remap onto the car to take advantage of the mods, however im not happy with it. I get a flat spot around 3500rpm and 4400pm, the boost gauge tells me that the boost is building but its very obvious when driving it that car almost feels like its struggling when it hits the flat spots. Only problem is it doesnt seem to happen all the time.

I put the car on a dyno dynamics dyno a couple of weeks ago and it made terrible power. 460 IIRC, however the setup wasnt great as they only had one fan at the front of the car, so cant really rely on the results. However it did show up a flat spot between 3-4k rpm.

I went back to the place where it was mapped and they suggested changing the N75 valve. Which i just got my hands on today. There is no boost leaks showing up on Durametrics.

Any suggestions?? would the N75 valve cause these issues??