Put the stock intake back on!!
seriously...that is whats causing you issue. the CAI is changing the velocity the air is traveling past the MAf...which makes the engine think it is running richer/leaner then it actually is. I have never had dealings with one on the STi, but on the regular WRX, it causes a lean run problem.
take the intake of...and reinstall your factory one, sans the resonator. same power gains, yet it doesnt mess with the MAF readings.
my "slightly" educated statements......