P0420 code what is the fix?

Question

initial MIL codes: random misfire, lean condition, o2 sensor input nonexistent, random misfire(#2). i have changed the o2 sensor(bank 1 sensor 1) cleared codes and now the engine stalls when accelerating. its a 2003 mitsubishi galant es 2.4L. changed the plugs, plug wires, plug boots, and one coil(found carbon trails and electrolosis ate a hole in the side of it where the plug wire plugs in). still stalls when accelerating. tested injectors with an ohm meter, tested fuel pump output pressure(50+ psi), tested TPS with volt meter(0.6-5.0V smooth transition from idle to WOT), removed vacuum line to fuel pressure regulator(no fuel present), tested manifold vacuum(20 "H2o), tested EGR with vacuum pump to diaphram(killed the engine at idle), EGR visually apears to operate properly(actuates artound 2000RPM). if you drive it easy no check engine light and runs fine, if you floor it, it wants to die and after driving it for a short time this way check engine light comes on but only with MIL of random misfire. if you drive it easy after quite a wile a MIL of P0420(catalyst threshold below efeciency). Im stumped. the only thing i can think of is a bad conection in the wiring somewhere but why no code?