2008 Subaru Outback downshift idle surge, CEL P0420

2008 Subaru Outback downshift idle surge, CEL P0420 As I slow down and the auto transmission (4EAT) downshift it seems to "hang up" a bit, then "let&go" and the car surges ahead a little tiny bit, not a huge deal but now it&CEL has come on twice, read it the first time as P0420 (and I assume it&the same now, I disconnected the battery Sunday night, cleared the code, now it&Friday and it has come back on). This code is catalytic converter related. O2sensor? exhaust leak? plugged converter? 146,000 miles on it. I&only owned it a couple weeks so quite a noobie.