1997 4 cylinder Toyota Tacoma Code P0420

Over a month ago I posted regarding the P0420 issue. Replaced CAT twice at muffler shop, last time more expensive CAT, drove 500 miles light came one, reset it. I thought I was home free after driving 823 more miles with no CEL. Yesterday I drove about 8 miles, turned off the engine, went in to the store, came back out, cranked up, and the CEL came on. Put my scanner on it and sure enough, P0420 bank 2. Why did it wait so long for the light to come back on? It would seem that if the CAT has gone bad again that the light would come on immediately? There were no other codes. People keep saying...go to the Toyota Stealership and pay more than the truck's worth to get original but I'm afraid if I do that, it still won't fix it?