bad catalytic convertor symptoms

i have a 2000 ss , and when i hold the gas at lets say 2300 rpm it seems liek it shunts up and down a few hundred rpm very slightly giving it the effect of having out of round tires, it also has power sometiems and other times its a dog, are these symptoms of a bad cat, also i have a bad cat code p0420, so i am wondering is it the 02 sensor thats dead ro usually is this a guaranteed cat....my rear valve cover has also started leaking along with mor eoil than normal aroudn the pcv valve.. which leads me along this route...has anybody else had similar symptoms and or cat trouble if so what were the symptoms

when the Cat went in my 2001 it would be chuggin like a MFer the think had no pickup or anything it just ddint wan tto go. the cat was plugged and acting like a JakeBrake. it couldnt get rid of the exhaust fast enough. When I plugged it into the scanner it showed the cylenters missfireing.
The cat should be under warentee and than ger a tune up- plugs, wires, fuel filter and an air filter. that fixed mine

ya i thought along those lines, i mean its going to the dealer tommorow, i do all work myself but if it is the cat they can do it for free....this car has gotten the whole tune up, and all that i never neglect any of my cars...the service engine light went off today, but who knows it may come back, we'll see when i commute to school tonight

well it was indeed the cat.....one symptom was when i started it every morning cold the idle would gor up like normal drop down up to normal and drop down then level off...