XJ 1995 4.0 Misses and stinky smoking Cat

My 1995 cherokee was running great after I replaced all the exxhaust from the head pipe back. All with a high flow system. A Crown Cat - 2.25 inlet and 2.5 outlet and pipe and a Dynomax muffler. That was about 2 weeks ago - only change I've done for 6 months. Except I gassed up at a different station than usual. Today it started missing at the beginning of acelleration and over the day got worse until it is missing all the time - like I pulled a plug wire. - checked - all connected. But the wierd thing is I opened the hood and burning smell and smoke poured out - - it was at the back of the engine and I traced it to the cat under the car - the smoke was from that and coming up into the engine compartment. So where do I start and has anyone been here done this? I plan to pull all the plugs and check them - I have no ohm meters or anything like that tocheck current.
Thanks

Cat overheating/plugged?

but it's less than 2 weeks old - can't get clogged that fast can it? and why woulld it overheat? too much fuel - or not burning all the fuel? BTW all plugs are clean and grayish brown. Well except the #1 which I can't for thelife of me get the connector off. I could if I broke it - but . . . . .