Ohh you meant the catalytic convertors. If they're clogged up will sound like poo and once car runs for a bit it will just turn off too much backpressure and it stalls out the engine kinda like stalling a plane engine.Then once you let car cool down (the pressure slowly releases) it will run again. Also you may have hard starting issues. Have you checked your enigne codes? Or at least put it on a computer to see if the convert is working? Of course with an offroad pipe you don't have to worry about that. Also with todays convertors you may have something else going on because they shouldn't get clogged up that easily.
thankfully it's not on my '03. It's the converter on my '94. I found a muffler shop in town and the guy revved the engine a few times, looked and listened. He stated that if the cat was clogged the RPMs would not be able to climb as fast as they did and then come back to idle as quickly.