The most problem of lambda sensors are the grounding....
original it's with 3 wires, 2 for heating, 1 for signal and the grounding by exhaust pipe.
you should buy a 4 poled sensor (with extra ground wire) and put the 4th wire to a really clean ground point.
Or you try by use a clamp (or so) to mount a separate ground wire to the hull of the sensor.
