2001 Toyota Camry Catalytic converter

For the past few months, I've been battling the following problem. My
"check engine" light came on in my 2001 Toyota Camry (4 cylinder). The
code, P0420, indicates that the catalytic converter needs to be
replaced. Okay, the people at the muffler shop say that the front
converter -- the one in the exhaust manifold needs to be replaced. I
do that. The light comes on again. This time, it's the same code, so
I get the rear converter -- the one near the muffler -- replaced. The
next day, my check engine light comes on again. What's the problem
with this car?! I'm at my wit's end. I keep being told that it's the
friggin' converter, but it MUST be something else! Everybody says my
oxygen sensors "seem to be working okay". What else can it be? Is my
on-board-computer corrupted or something?!

HELP!