help power loss

[lilkoreanboy]

i got a 91 tsi awd turbo car with apexi 3 inch exhaust. i was wondering when i punch my car at a low rpm and starts to get to 4000 it starts to lag power becomes slower but sometimes it wont do that it just pulls through. most of the time its been lagging though. (its not boost leak)
PLEASE HELP

 

[DSM90AWD]

Please fill out your vehicle profile so can see what other mods you run and better help.
Also list anything relevant to your issue, i.e. I added/replaced/modified "X" and now runs like crap.