my car has suddenly started over boosting like a bitch and cuts out EVERY time it idle's?

i noticed summat was up when it started wheelspinning when it came onto boost in 4th

ive got a bailey dv26 dump valve on it (yes yes i know there shit and sometimes cause the car to cut out, and its coming off very soon )....but surely it shouldnt cause it to overboost

cheers, andy