The problem could be wiring......dirty connection.....PCM (less likely) or the oxygen sensor.
With the downstream sensors.....it is not so critical to match them....so you would be fine changing just that sensor.
The downstream sensors switch much slower, and the only thing they are used for is to monitor the catalytic converter to make sure that it is working.
If it were an upstream sensor.......as those switch MUCH faster.....and are used for air/fuel mixture adjustment, and add in the fact that oxygen sensors' switching speed slows down slightly as they age........I personally like to change BOTH upstream sensors if 1 needs to be changed so that you end up with both sensors with identical switching charactoristics.
In your case....I would really carefully examine the wiring to see if you might have a loose connection, or wires against the hot exhaust system.
If you don't find anything, you could switch downstream sensors from side to side.....clear the codes (remove negative battery terminal and turn headlights ON and wait about 5 min)....and see if the code comes back......check the code and see if the problem moved with the sensor or stayed the same.
This will tell you if the problem is the sensor or the wiring, etc.
If you have one of those downstream sensors mounted into the back of the catalytic converter body (like one was on my '96) be super careful as when I was changing the "Y" pipe on my vehicle.....the whole threaded part came right out of my catalytic converter body with very little turning only pressure.
That would be a problem....as the metal there is very thin.....and a "Y" pipe is pricey.