You are hitting boost cut...
Either get an aftermarket ecu or an safc or similar piggyback ecu...
The car is basiclly cutting out due to an overboost error, the ecu is trying to save the engine...
So when you turn the boost down it does not cut out, and you have no power because you turned the boost down....
Also search topics "Boost cut defender, fuel cut & boost cut"
There are many ways to combat it with out spending allot of money, but if you have a stock ecu id spend the money personally and buy a PFC straight away. Its not safe to run 12psi on a stock ecu with standard fuel maps...