12s out of the turbo setup will require a lot of upgrades to the system. i would check out the fuel pump, i had a problem like that where it would stumble and buck like a bronco except when boost would come on, and i just sprayed the pump with battery cleaner and problem went away. i previously degreased the rear suspension and got dirt on the leads im sure. i have to run a bov with a line back to near the turbo on my setup as well, i remember when venting to atmosphere it would automatically stall out as soon as i got ready to shift into the next gear.
im running a bosch bov, and it vents at idle, and has to be routed back behind the afm to not stall out the car. ive had a lot of stumbling issues to get the car to run right, but its cool now, if not a little bit slow to me, but i dont care, its a awesome driver and thats what i do, i drive, a lot. once i take care of other issues not car related, im gonna make it a better driver, and turn up the boost to about 10psi with an ic and drive a little faster,
btw. one of my buddies ran 13.4 @108 with a stock turbo setup, starion intercooler, and 15psi and no traction-i didnt see it but i believe him, hes a good mechanic and trustworthy.