2001 Tacoma 4wd 2.7L Oxgen Sensor throws code in damp or rain

I own a 2001 Extended cab Taco with 4WD and 2.7L with 305,000 miles. Both catalytic convertors are less than 2 years old as are both exhaust sensors. At 220,000, it had a valve job. The Taco burns a little oil but a little less that 1,000 miles per quart.

My problem is that whenever the air is damp or there is rain, the CEL goes on frequently and it is always the P0420, low cat efficiency. It always trips when it's raining etc and never trips otherwise.

I bought the current rear sensor from Rock Auto. Recently, when I was at the Toyota dealership, I asked the parts guy if he had an opinion and he said the parts you get on Rock Auto and E-Bay may have the same part number but are not made to the same specs as Toyota supplied parts.

Has anyone else encountered this and what does the community think?

Thanks.