3rd upstreem 02 sensor fail in 6 weeks??

It was replaced on my 96' just before my purchase by previous owner ( I didn't know)and then by me a month later. Now the code is up again. Tells me that it's reading slow. I'm guessing that it's not the o2 sensor afterall. Any experience /thoughts? I'm thinking cat converter? Other possable causes? This is driving me crazy!

oh yea, this last time , i also got a 4cyl missfire code that points to the cat too.

Any input appreciated.