I found that when i replaced my sensor it ran worse than before. Turns out that those sensors are quite sensitive to the wire length and any difference in resistance. Which makes sense considering thats how the sensor operates anyway, so I probobly should have thought of that when I bought it. I replaced mine with the oem style and it worked quite a bit better. Not sure if that would actually cause the problem but two problems could be arising. One is that as the engine bay heats up the wire for the o2 sensor (if its cheaply made) could change resistance, though it shouldn't and that idea is a bit of a stretch i think. Also the ecu only takes readings from the o2 sensor after it is up to temp, since the sensor doesn't function cold. I'm not sure what the time delay/activation temp is. Quick test though would be to simply disconnect the sensor and see if anything changes. May have to reset the ecu after you reconnect it though.