Thanks for the replies guys.
mad082, I don't hit the boost often since the car is non-turbo but thank you for the reply, I usually shift the gears around 2.5k's RPM's, I don't really drive hard.
I found a thread on how to test the O2 sensor with the paperclip method, so I will do this on the weekend. Only mods I have on the car is an exhaust system and splitfire coilpacks, I'll test the o2 sensor on the weekend, if that's fine, then I will try changing the coilpacks.