Nox is created by high combustion temperatues,usually caused by lean mixture.Replacing the Cat may get the vehicle to pass for now but you're not addressing the root cause.Try one of those BG intake system cleaners and make sure the hot air door in the air cleaner is closing the whole way as both of these could also cause high NOX.If the co% reading was way low chances are it's too lean,what HC reading did you have ? possibly a lean misfire is the cause of the stumble.

I failed NOX also, but by a much larger amount. Replaced the cat and passed. But before you do that, do a search on the site for NOX and Chrysler Ignition. You will find some good info and a few positive reports. Might be able to pass by just removing and plugging a hose.

There's some some emissions info in the FAQ. Also check out this article for good background info: RWD - Real-World Repairs for Real-World NOX Failures

If you are able to adjust timing, I'd be inclined to check to ensure it's not overly-advanced.

Otherwise it could be the cat. I just took my Volvo through and 9ppm on NOx. No spiffy mods and regular octane gas.
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Norm Cook Vancouver BC; 1989 745T 204,000KM