p0420 code keeps poping up on my 03 3.4 230k. but now my car, on acceleration chugs and cuts out on me when starting from a stop. it idles fine and when i start to drive the car it will drive fine for a while and then it will start chugging even at 55mph and i have to let off gas and accelerate slowly or i floor it and it will chug alot before it quits acting up. it doesnt do it all the time it does it every other time i drive it or after i have been driving for a half hour or so. some times it wont do it for a few hours and a few stops, some days its fine and some it is just horrible. the only code i got is p0420. my o2 sensors are functioning properly according to my computer. also in the same system is the catalytic converter. i was wanting to know if a bad cat actually would make the car chug like that. and or could my trans be making the engine do this? my trans slips a little bit here n there.