on a stage 1 flash the target boost is 12 pounds on the ACN 91 map, and 14.5 +or-1psi. whats the exact map flash you have? also things to check are
-vacuum lines around the boost controller \ waste gate
-make sure restrictor pill is present
-test the boost solenoid make sure its functional
also are running the stock wrx top mount? i had a similar issue with the stock coupler, it had a tiny crack in it.