update....Ive just collected it from the rolling road and I think I know what the problem is. He said it was doing it sometimes but not all the time. When I gave it some on the way back sometimes it would hit 22psi but other times it would only go to about 16 or 17psi and then not boost anymore and jolt etc..., as this was going on I could hear some hissing from the dump valve area so I wreckon the boost was pushing the piston up in the dump valve and boost was leaking out, anyway I'm going to blank it off and see what happens !!! report back soon