Shudder under boost

[Mexik]

So recently my s14 has started to shudder under high boost. When im driving everything will be fine but when I make high boost (from 12-17psi) the rear end of the car starts to vibrate heavily, I feel a power loss as if I've lost all boost pressure and the car revs


Mod list:
Hks stage 1 cams
Nistune ecu
Os giken super single plate clutch
Garret 2876r ball bearing turbo
Stainless manifold
Braided turbo lines
Greddy intake pipe
3" turbo back exhaust with x force dump
1000cc injectors
Z32 afm
Gtr fuel pump
HDI triple core fmic
Turbosmart boost tee
Gk tech clear CAS
Gk tech rocker arm stoppers
Upgraded oil pick up
Oversized baffled sump
Full sized alloy rad with Thermo fans

Suspension/brakes
D2 8 piston front calipers
D2 330mm slotted rotors
S14 2 piston rear brakes
Ultra racing coilovers camber/height/damper
Adjust rear toe arms, traction arms and camber arms
R200 3.9:1 shimmed LSD diff
Gk tech solid cradle bushes
Gk tech solid diff bushes
Rose joint caster arms
Rose joint tie rod ends

[EarlsWorld]

Are you sure it is coming from the rear of the car? could the vibrations be mid/front possibly?