I have an Astra Gsi Turbo. Just noticed the other day a vacumm pipe had come off the rear boost control valve, its was the L shaped pipe. Ever since putting it back on the has been cutting out when you slow down to stop at traffic lights ect.. Is starts fine but when you blip the throttle as the revs drop back down it cuts out. Also when driving around when the car comes on boost it seems to splutter and sounds like its missfiring? Also recently fitted a collins dump valve, the twin piston type not sure if this would make a difference?