You may have the gain set too high and it is bouncing around under full boost. I have the Greddy Profec Type S, which is a redo of the old Profec B. If I jack up the gain it bounces around at the desired boost level and does feel like it hits a wall.

 

You didn't say the full name off the BC, but if it's the SBC type s. Your problem might be that your warning is set too low. And what happens is that lets say its set to 1.1 bar (16 psi) when you hit the set warning of 1.1 or so it will automatically lower your boost by certain percent that you set. For example I have the type r sbc and mine is set to 1.5 warning, which means if I hit 1.5 bar it will automatically drop my boost by 20% as I want so I don't hit fuel cut, because I don't have a flash yet.