I have just put a highflowed T3/T4 hybrid turbo (57mm front wheel, 60mm rear wheel) on my car and am not running any boost controller and it sits on a constant 8-9psi but high up in the rev range it just seems to start to make a heap of boost all of a sudden. I am still running the standard cast dump pipe off it as well. Any ideas what could be the cause?

waste gate cannot flow enough to control it.