Ok i'm hoping for some good idea's here as I have tried everything I can think of!
This is an intermittent fault so sometimes it's fine other times it's not but it will do it everytime I drive so it's a problem that needs sorting!

Start accelerating and as the turbo spools up it will not hold boost, the boost gauge needle will flap around something crazy, the car will splutter, backfire and jerk around as the boost tries to increase but doesnt in a on/off type fassion.
It feels like a boost hose has come off or there is a big boost leak some where?!

The car is MFI with Bailey breather setup and metering unit return blocked venting to atmosphere, I have tried.........

Removing every boost/vacuum hose and checking for splits, twice! (all hoses are Samco and done about 5000miles).
Brand New Turbo/Manifold/Garrett -31 Actuator.
Brand New Apexi AVC-R Boost controller (fault is still there if i remove it and pipe turbo direct to actuator).
New NMS Head.
2x Different Cam/lifters (one was a brand new set and ran in properly).
New Plugs, leads, Dizzy Cap, Rotor Arm, Dizzy, Coil, ECU's (Ignition and Fuel).
New Fuel Filter.
Voltage to Coil and Fuel Pump is 12.5v.
Timing.
Disconnecting boost gauge.
No Dump Valve.

So i'm left thinking, cracked inlet manifold, charge carrier pipe, throttle body, or maybe an injector not seating correctly?
Any and all idea's greatly appreciated!!