03xb9 needs to be check out, I don't know too much about it, gone thru a set of plugs and it shut off on me the other night, it has the stage 1 kit of w.e..
Honestly though, i think you need to start off with ECM Spy. Do a TPS Reset and flash your ECM with a new Race Tune. The Previous owner told you it has the race ECM, maybe he used ECM spy, created his own "custom" tune, which sucks.