Audia3 1.4 turbo underboost

[Swoop72]

Got a p2099 underboost code .this as done my head in .all new sensors .new d/v fitted .smoke tested no leek .wastegate problem found by mechanic. New turbo fitted .mechanic clears codes .2 days later code is back .car as a slight lag / hesitation around 3 to 4 k rpm .i have had a remap and a custom de.cat which is 3 inch stand it was 2.quarter .any ideas anyone or could it be the decat is to big