Which cat did you have replaced? The V6 has one in front and one in the back of the engine, then another further back along the exhaust line. The P0420 code comes on when the values from the two O2 sensors on the Bank 1 cat are very close to the same (basically a differential between intake/output of the cat). Since it's only one, not both, there's a chance the two O2 sensors aren't connected back on that new cat. Just a possibility of course. I've seen weirder things after coming back from the shop that took a few weeks to show up.
*Edit* Just saw the writeup that you had some stuttering and possible misfire issues. Depending on how bad the cat was that you replaced 4 months ago, it's possible the other one is simply failing as well.
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