1.8t boost problem
 

HI

i just recently bought an a3 1.8t with modifications-
atp top mount turbo manifold
gt28rs turbo
uprated fuel pump and injectors
apr dv
custom map mrctuning
maybe more that im unaware of
running over 300bhp

i bought it with a blown turbo and fitted a new one, the problem is boost! it starts boosting at around 4000 and seems ok in 1st 2nd as im feeding the power on but through the other gears it starts boosting at 4000 as normal but past 5000 it feels like the boost is sputtering on and off, there is some backfiring as it does it, ANY IDEAS!!!! its started to get very frustrating looking for problems