2000 impala ls keeps stalling out

So, I first notice this problem every once in awhile. There would be no notice. Usually when I had to leave to do something really important. I would try to start the car. It sounds like it wants to start, but won't turn over. Like it isn't getting a spark or won't connect. I would wait about 20 min to an hour. Try again and the car is fine. No weird noise, smell or anything else. .. the car just stalls with no warning. Waited about 20 min. started back up.. drove about 2 blocks and stalled again. just put in new catalytic converter but check engine light still on and code is reading p0420 can you please give me a solution