P0420 code, car stalling

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Hello, I have a 2003 subaru forester with 166000 miles on it. A few months back the check engine light came on, but the car was running fine. Took it to a mechanic and a P0420 code came up, he said if the car is running fine to just keep an eye on it. The light turned off and the car continued to drive fine for 6 weeks. For the last 2 weeks, I've noticed the car has been stalling when stopping and at intersections. Today the car completely shut off at a light, I turned it off and then back on and it ran fine. I also checked the code today an P0420 came up again(check engine and cruise control light on). I will get it checked this week, but it seems like many people with this problem need a new catalytic converter. Any idea what can be causing this problem and what the repair cost may be?