Update - As many indicated was a probable cause, I think my problem is tied to issues with the wire harness connectors that the rear O2 sensors plug into. I unplugged my Driver's side rear O2 sensor and plugged it in again firmly. Started the vehicle and the Ultragauge showed the sensor was oscillating in alignment with the rear O2 sensor that was not throwing codes again. Started driving, hit a few bumps in the road and the Driver's side O2 sensor stops oscillating again indicating connection problems internal to the harness connector. Thanks to all who pointed me towards issues with the harness connectors.

Anyone know of a good source for replacement connectors for the wire harness side connectors?

 

I bought a used engine harness of ebay for 2002ish range rover...had plenty of good spare sensor connectors which fit (not the whole harness, just cut off and solder on as needed), for low price....not new, but worked...