Troubleshooting tuning problem or mechanical problem?

So this question is related to my 2007 STI. I bought it already pretty substantially modified so it's running on the tune that came with it until I can afford to change the setup a bit. The problem in question is the engine likes to stumble between shifts when the blow off valve activates and suddenly coming back on throttle. Occasionally if i drive it harder than that I will just lose engine power all together for several seconds until it decides it's going to continue running. If I take it to neutral during that time it will, in fact, just die. I've been chasing my tail trying to figure out if it's fuel, ignition, vaccuum or some sort of tuning issue. It never throws a code so I'm kinda stuck with how to go about finding the cause of fhe fault. It's running a cobb V3 accessport.

That sounds like a tuning issue to me. Is the blow off valve vented to atmosphere?

Yes. It's a vent to atmosphere BOV. I do know for a fact that the BOV is not closed fully at idle and it lets air out so I'm thinking the BOV may be causing issues of some sort but I don't know if it's related the the specific issue above. The BOV is a Tial Q style. It's pretty old like the rest of the equipment on my build as I bought it already modified and I'm currently just trying to track down the cause of all the little issues I'm having before I start throwing parts at it. I would like to have a mechanically sound car before putting a tuner through the headache of having to chase their tail trying to tune it with things that aren't functioning properly.

Air flow meter based ecus generally don't like atmospherically vented bovs I would plumb it back in.

Car is running Speed Density. I'll attach my mod list to this post below.

ENGINE:

-Real Street built 2.5l

-CP Pistons

-Manley H-beam rods

-ACL Race bearings

-Ported heads

-Ferrea Dual Valve springs/Retainers

-Ferrea competition oversized valves

-Cobb AccessPort V3

TO DO:

-Replace Timing Belt

-Replace Water Pump

-Replace Thermostat

DRIVETRAIN:

-ACT 6 puck clutch

-Kartboy short shifter W/ TiC bushings

-Group N Transmission mount

-Beatrush Pitchstop Dogbone

TO DO:

-Install Driveshaft Shop CF driveshaft

-Fix axle boot passenger side

-Replace/upgrade clutch

AIR INDUCTION:

-RSP Rotated 6262 T4 .68 V-band Turbo kit

-TGV deletes (butterfly delete D:)

-Ported intake manifold

-Tial Q BOV(needs replaced)

-Custom 3" Intercooler

-Coated intercooler pipes

TO DO:

-Extrude Hone STI Intake Manifold

-Upgrade Intercooler and piping

-IAG TGV Deletes

-Upgrade Turbo to GTX2876/GTX30XX/GTX35XX

EXHAUST:

-Tial 38mm wastegate(upgrade to larger)

-Helix mid-pipe

-WBR 3" Axle back

TO DO:

-Upgrade Wastegate

-Killer B Holy Header

-Custom Downpipe? + new exhaust

FUELING:

-Perrin fuel rails

-1000cc injectors

-Denso Supra 300lph fuel pump

TO DO:

-Cobb Flex Fuel Kit

-Upgrade to ID 1700cc Injectors

-Upgrade Fuel Pump

-Tidy up wiring for fuel system

-Upgrade Fuel Pressure Regulator

-Upgrade Fuel Rails (IAG)

-Radium Engineering Surge Tank + Dampers

-Install New Fuel Tank O-rings

-Find fuel smell

SUSPENSION:

-[Feal Max Travel Coilovers (6k/4k Swift springs) Whiteline complete set of suspension bushings November 2019]

-Eibach camber bolts

-Whiteline 27mm front sway bar

-Whiteline 29mm rear sway bar

-Kartboy rear endlinks

TO DO:

-Install Feal Coilovers

-Replace all suspension bushings with new Whiteline bushings

INTERIOR:

-ATI Triple gauge pod dash mount

-STRi coolant/Oil pressure

-AEM Uego wideband

TO DO:

-Upgrade to newest gen steering wheel

-Upgrade stereo system

-Custom modify center console

-Update Gauges

CHASSIS:

-Stoptech Braided Stainless Brake Lines

-Perrin Master Cylinder Brace

-STI Foglight Covers

-Hella Sharptone Horns

-Grimmspeed Hood Struts

TO DO:

-Rebuild Brembo Calipers

-Replace Rotors

-Finish Installing sound dampening on doors and misc areas.

-Install IAG AOS

-APR GTC-500 Wing

-APR Front Splitter

-Seibon CF Scoopless Hood

-Underbody full tray