Hi my rs turbo has an issue with boost. It boosts lovely on 1st 2nd and 3rd but when you get to 60-70mph in 4th or 5th it struggles like its hitting a wall then suddenly it will drop revs to 0 then do a massive back fire then engine with go back to normal and go back to the revs it should be. Just wont boost properly. Did it all before head gasket was done and also its setup. What else can it be now? Cheers in advance