Lee - I have EXACTLY the same probem at the moment with my new engine. It only runs 6psi due to not being setup. Mine doesnt splutter though - just holds back as soon as it starts to boost. It's either the knock sensor or the boost hose - i have a rubber one that Gus's mate kindly gave me - it feels quite firm but still think it's this. Going to swap it over - anyine local to u got a hose u can swap over?