Sounds like no tune, and some one modifying their car and not knowing what they are doing. You really either want to not drive the car (especially in boost), or take of the down pipe, or get the thing tuned. Your car will not run right without doing one of those last options, plain and simple. But getting the code read will also tell you the exact problem. Your car is going into limp mode cause of the new free'r flowing exhaust you just put on and the car isnt liking it much.
"2007 WRX- PPG'd, 20G- Tim Bailey tuned
Stage 2 2005 WRX