I had a new O2 sensor installed today,I got a OEM-style made by Walker(from AutohausAZ),it cost $78 incl shipping,and a local mechanic put it in for $20. It seems to have cured the flat spot in the acceleration curve I was experiencing,I will monitor MPG to see if that increases.
The Walker O2S was built better than the original Denso sensor;the Walker had a much better sleeving over the sensor wires,and came with copper hi- temp anti-seize compound in a tiny tube.