hrm, that is awefully strange.... Do you think that you might have too much backpressure from your exhaust? Some how I have this dumb feeling that you might be looking at having to replace your injector. Do you have any exhaust leaks anywhere that might contribute to a strange o2 sensor not working properly? does it burn oil, because if it does, then it will carbonize the o2 sensor and it could relay wrong information to the ECU and return change your fuel to air ratio. I would suggest starting witch checking your 02 sensor