Yeah mate this sounds like a tuning issue to me if the injector pump has a boost compensator this will give you the symptons your describing as it wont open to allow fuel until you have boost and you cant make decent boost until you have fuel. The turbo and fuel system run hand in hand getting yours rebuilt with what you mentioned will basically be a high flowed version of your current turbo. You can make the standard ones good but the aftermarket items although of the shelf are much better suited than that tiny little ht18. Keep the turbo and find a better tuner ask them to check the timing and basically tune the compensator out of it then go from there.