max boost is for high quality 93 octane+ , if you use lower quality fuel, this could lead to a choppy throttle
Another reason could be the spark plugs, if they are going bad, they cant handle high boost. Eventually this will start happening with lower boost as well.
Its not going to be easy, i recommend you get the parts from a US dealer (we tend to offer better pricing) and take it to a reliable mechanic.
All the parts we offer are direct stock replacement, you do not need a tuner shop to install, just a good mechanic.
I wish you recorded it so we can check exactly what is going on and the difference. I use open intakes in Vegas on my car and im pushing ~ 700 HP , with no issues whatsoever