2003 Civic EX Intermitten Stalling

My Civic has a (very) intermittent stalling problem. It's happened five times in the past year. It seems to stall only when it is hot and humid, when the engine has been running for a long time (45 minutes or longer), and usually when the engine is under a large load (A/C running, idling for a long time).

Before it stalls, it starts to sputter and the gas pedal seems to only work every few beats instead of continuously. I normally have enough time to find a convenient place to park and stop the car completely before it dies. Other than when it stalls, the car runs perfectly fine.

I have replaced the fuel filter (not the pump) and the spark plugs, neither of which have helped. The check engine light has been on for a few months (P0420, catalyst efficiency below threshold), but it stalled out twice before that code ever came on.

Do y'all have any advice on where to go from here, or how to diagnose the problem directly? Thank you very much for any help.

Next time it stalls. If it won't re-start, you need to check for spark. If spark, suspect fuel delivery. If no spark, suspect ignition issue.