do you still have the stock 4.2 cat?? it could be inadequate, but you say it is missing. soo.... it has to be REALLY reach to miss or really lean. if you aren't seeing any black smoke i bet you are lean missing. you could get one of those cheap AFR gauges at autozone or hook up a voltmeter to the O2 sensor to see what it's feeding back. if the voltage is correct at the O2 sensor it might be bad. the best bet is to get some 5 gas readings though... do you have a JC or regualr college around?? they'd probably take a look at it for you for free and they'll have a 5 gas machine. also a lean miss can overheat the cat and make it burn out. first off i'd get some numbers. high CO means rich Low or 0 CO means lean. if it's only started in the past coulpe hundered miles and hasn't done it before, it means some part has failed. don't let them bursh you off, saying its normal for a new engine to miss.