I say is your cat I had the same thing bad O2 make the unburned gas go in the cat then it eats it away slowly so make sure you check your O2 every time you tune up that goes for all of you.
The inside is busted and a chunk of it turned to the side blocking the air flow
so after I bought a after market one, it only lasted 2 months tops if that.
Go with OE find one from the junk yard buy a new O2, when you buy one from the junk knock on it and shake the shit out of it make sure the inside is not lose nothing falls out of it (crums etc)
DREMEL is South Africa Engineer's best friend
<span style='color:blue'>HID all the way </span>