RRE

I have a small problem. It seems that my car have a problem dealing with low revs, that beeing below 2300 RPM. It has slight problems when you press the throttle just a little, it you floor the pedal, no problems. And as soon at you reach abowe 2500 RPMs the problems gone. The pressure is fine and stable to.
It's a '99 Impreza GT, de-cat turbo back HKS Super Drager, K&N induction, HKS iridium plugs, brand new MAF and JB ECUTEK.
Could it be the lambda probe?