Possible clogged catalytic converter

They usually collapse, this looks like a clog but the matrix of the converter has collapsed and causes a blockage, the cat will have to be replaced, age can be a factor, over rich mixture due to a miss fire can over heat the cat and "cook it"

Just to add to this one I had a feeling you had a clogged cat when you said in the question you asked the other day that you had to let up on the gas pedal for it to shift. So did you have a back pressure test performed to find this out?Which one is it?The most common one to clog up is the one towards the firewall the Y pipe one.