P0420 still after new cats & o2 & egr

Question

I replaced my egr valve, spark plugs, both catalytic converters bank 1 & 2, and a new oxygen sensor on bank 1. Light stays of for a while any where between 50-145 miles it comes back on again, same code P0420. Could it be the evap canister out the lines too and from the tank? I swear I have tried everything I really don't know what more I could do. Help!!?!??!??