WhiteSTi3

Just by the Turbo on my 2001 WRX UK I have a silver hexagonal cylinder with a hexagonal nut on top with a spring between them. It is adjustable clockwise & counter-clockwise & has vacuum pipes going through it. I am guessing that it may be some kind of boost controller? if it is, how do I know how to set it or if it is set correctly? I am getting a hesitation from the engine when accelerating in 3rd gear upwards, so just looking for things that it could be. The Car also has a Powerchip & FSE Boost controller (Which is for fuel I think) Any Ideas?