I might have missed something in that first post, but I don't see a question there, just a statement of what is approximately normal operation.
Max listed a couple possibilities related to the 0172, and those are fairly easy to check.
Another possibility is that although the sensors are responding correctly, the typical failure mode for a zirconia type oxygen sensor is to report a decreasing EGO so that the PCM keeps leaning out the mix until it exceeds the fault tolerance. Do you have a gas analyzer to actually check the EGO and confirm the sensors are not weak?