Boost issue mk1.5

[nik]

so lately i noticed i have a weird thing going on when accelerating.
i initially thought it was ignition related maybe coil or plugs but noticed today that my boost pressure is showing a problem.
i took a vid of the gauge under boost conditions and you can see the needle will drop on occasions for a couple of psi..you can really feel this in the car.
i dont hear any other noises or see any fault codes.
any ideas?
first part of vid is at 16 psi then after a minute it switches to 18 psi.



for ref the number at 10 o'clock position is 10psi and 20psi is at roughly 12 o'clock.