Hard starting after sitting, code P0420

It starts fine in the morning and after a few minutes of sitting, but after a few hours (3-4), it cranks but doesn't want to engage. When it finally does, it splutters as if you flooded a carburetor on an older car, and there's a strong fuel smell. It's becoming a frequent problem. Code scanner lists P0420, but the catalytic converter is only a couple years old, as is the bank 1 O2 sensor. Also, I think this is just a symptom and not the cause? I have read about pressure regulator, pump, leaky injectors, etc, but none of the symptoms seem to quite fit what people describe.