WE2RX7,
I don't think its the O2 sensor..
I cleaned the MAF saturday morning and the car still idled like shit (cold start). But when warm it cranks and starts fine.
I the put the HKS twin power on Sunday and during cold start, the idle was horrible! It would hunt and then couple of times the engine will just shut down. I'm going to get a new O2 sensor and ground the fuck out of the engine
Regarding HKS... I noticed slight more power out of my butt dyno. Car also seem bit more responsive to the throttle.