Don't know if this will apply for you, but here in the Midwest where Ethanol blended gas is the norm, I had lots of problems with downstream O2 sensors over the years until I stopped using Ethanol blended gas. After I stopped, my O2 sensor problems went away.
They aren't very costly and if you have the right special socket, they are pretty easy to replace most of the time, especially on newer vehicles where they haven't had a chance to get frozen in place.