Just had a Cat with extremely strange symptoms. Engine would run good ( at any given rpm), then suddenly die off, some times to a stall, other times it would catch at the last moment, then run good again. There was no rhythm to it though- turned out to be a faulty PCV valve. I thought of your situation. I simply removed the PCV valve, plugged the hose & ran the engine- symptoms went away. This is of course after checking all the usual suspects & then pulling my hair out- this really stumped me- never saw a PCV do that before