You know what buddy, I drive the same car as you and I'm also a proud Auto owner too... guess what, the issue may be issue then you think...
if you're running an atmospheric BOV - which means it vents 100% into the atmospher without plumbing back then your car will stall. R33's and most other nissans have airflow metres that directly works with the engine ECU... if the ECU detects a change in airflow it will also affect the mixtures of fuel, air, volts etc and hence will cause your car to stall as a precaution mechanism.
Just get a stock blow off valve put back on - plumb it all back.... reset the ECU (easiest why is by unplugging the battery for 2 minutes then replug) and then turn the car on, let it idle for 10 minutes... then drive off and it should be fixed... simple, I know... I was shitting myself too... but this is the quickest fix.
Good luck - if it's not this and your mechanic's given up - then Bob's your uncle.