1mpreza

Hi guys,,, im new to this site and impreza's im loveing my MY/99 but have constant problem at low boost [hesitation, stuttering] but runs great on hight boost, i knew it had a manual boost controller and just thought it was a normal thing to mod in turbo'd cars..... but just done a bit of research and turns out there really bad for my scoob I've changed the plugs, HT leads, fuel filter,cleaned the MAF and reset the ECU,,,, is there a chance this crappy boost thing is the probplem ?? the car has a full decat which aparently help's if you have a MBC ? Anyway the car isnt going anywhere untill i've removed it,,, but im having trouble finding if and what i have to replace it with ?? Any help would be great