the O2 sensors do kick in when the cats get warm, so they could be to blame. and they dont always throw a code. if you havent replaced them in the last 50K, do it and be done with it.
the system could "think" its running too rich, so it leans it out to the point where its not combusting properly. i would use something that measures all the DME inputs at once, and see which one goes nuts when the engine freaks out.
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