Welcome to the Club
It's extremely rare, but a plugged catalytic converter could do this, but I seriously doubt it as you would not be starting up the vehicle at all or very little.
It sounds to me more like you may need to clean the IACV valve, http://www.clublexus.com/forums/rx-f...-write-up.html
That's usually one of the main culprits for causing the vehicle to shut off at idle. I am thinking the P0420 is showing up as the vehicle is not getting enough air, causing a mismatch in the air/fuel ratio causing the P0420.