S13 SR20DET idle problems on cold start

[zboost]

My S13 which is red top SR20DET has idle problems.
It just started happening shortly after I swapped my FGK Legalis R turbo back for a HKS Hi power system.
I am unsure if this is related to the idle problem but is it possible because I didn't use a gasket from the downpipe to the cat delete pipe?

When I start her up in the morning idle fluctuates and has random surges where it drops to the point where it's about to stall and then raises back up or actually

stalls.
This continues to happen until the cars fully warmed up.
While driving as I come to a stop the car tries to stall and does unless I give it throttle as I brake.

Checked the spark plugs which are HKS branded, the tops were completely black (done around 15,000-20,000km since changed). I swapped them out for the previous HKS

spark plugs that were in, as they were atleast better than what was currently in there until I'm able to get new ones.

Injector/s are clicking, not sure if this is related to the idle problem.

Looked for vacuum leaks, couldn't find any but I might not have looked properly. If it shows hitting full boost on the boost gauge does that mean no leaks?
The genuine HKS bov appears to be leaking boost as you can hear it before it even hits +vac on the boost gauge, this has been happening long before the idle problem.

Ran new silicone vacuums for the boost controller & wastegate

Just had a new Exedy 5 puk clutch put in around 2000kms ago
Car was serviced 5,500kms ago

Engine mods:
HKS Super Fire Racing plugs
Factory injectors & coil packs
HKS Hi Power exhaust system
Cat Delete
HKS wastegate actuator
HKS SSQV Bov
GReddy Profec B Electronic Boost Controller - 8.5psi low & 14psi high
K&N Panel filter
Exedy 5 Puk Heavy Duty clutch