Hi everyone, I have a boosting issue and have no clue what the cause could be Looking for other peoples input or advice.

What the issue is:

STOCK ECU, JDM S15 ENGINE, boost to 0.7/8 then clicks down to 0.5. No idea what could be causing this.

What I have done so far:
Reset ECU(by disconnection battery..)
Take off the turbo, Retighten everything.
Made the HKS Wastegate actuator tighter.
Done many boost leak tests which seems to drop slowly.. (Is it supposed to hold the pressure?)
Changed the vacuum hoses under the intake manifold.
Changed intake manifold gasket(the top half).

Info About the car:
Blitz FMIC
Walbro 255
HKS Wastegate Actuator
Hks mushroom air filter
Stock T28BB.
Always used 99ron

What I have found out:
I have a gizzmo boost controller in place which I can increase the boost after the click down. But the running the stock controller (wastegate solenoid) gives the same result. it boost to 0.7/8 then clicks straight down to 0.5

I would like to use the original boost control solenoid. without the issue that is present and have the gizzmo there purely for reading the digital boost pressure readout...

I am pretty sure JDM S15 engines are supposed to run around 0.7/8 BAR of boost with stock ecu and solenoid valve. or at least hold that much with ease.

I don't know if it something to do with fuelling as I have also removed and cleaned the injectors(brick brown). checked the spark plugs(BKR7EIX), they all look fine.

Link to VIDEO with ISSUE: https://www.youtube.com/shorts/Zckumv2mPL8

I hope you guys can help out...