O2 sensor? Sounds alot like what mine went through when the wires severed themselves. Used that as an excuse to get a stainless elbow and 300zx style o2. Try inspecting the wires going to said o2 sensor. I ended up unplugging mine as a diagnostic "tool" to see if it made any difference in drivability since under WOT, it basically ignores the o2 sensor anyway if I remember correctly.