Code P0420, post catalytic converter O2 voltage

I'm trying to figure out if it's caused by a bad catalytic converter or a bad sensor. The 2 upstream sensors (bank1+bank2) follow each other almost identically, varying ~.6-.7V at idle and with some gentle punches on the accelerator pedal. The post catalytic converter O2 voltage swings wildly. After the engine is warm (driven 30 minutes), the B1S2 will hover ~.7V at idle, but drop to ~-0v when the accelerator is gently punched. Does this sound normal ? The P0420 takes about a week to re-surface after a clear.