Ok the problem is fixed... strangely enough it was to do with the throttle body. I took it off and gave it full clean, noticed that the bolts were a little on the loose side, also cleaned the plugs with WD40 then blew them out with the air compressor. took off the TPS and also cleaned in there. put her all back together and made sure all the bolts and connections were done up properly and now she boosts right through all gears. such an easy fix that didn't cost a thing and only took about an hour of my time.