2002 Chevy Impala P420 Code-Change Oil Message and Check En

I bought a 2002 Chevy Impala about 2 weeks ago. The engine I was told was replaced with a 60,000 mile engine. The check engine light was on and after the O2 sensors and battery were replaced there were no messages or lights. The next day both came back on. I had the oil changed and the message went off. The very next day, the message was on again. I took to a personal mechanic who ran a diagnostic and the code P420-Catalyst System Efficiency Below Threshold came up. The mechanic told me that the mesh in the threshold was probably near gone or gone. He also said that the previous engine could have possibly missfired and caused this. I was also going to get a tune up because the car stalls/cuts off after trying to stop and accelerate soon after. This has put me in a dangerous situation while driving. Is the stalling /cut off caused by this code?What are your thoughts?