Toyota Repair: 1999 Solara Sputter-Jerk, Code P0420

Question
I have a 1999 Toyota Solara, have owned it for 13 years. The car has started to sputter when in drive and sitting on the brakes. On acceleration from a stop, the car will sometimes jerk and/or hesitate. On the open road it drives fine and accelerates fine. When and OBD-II reader is used, the code P0420 is presented. Check Engine and Trac Off light are on. What are possible causes?

Answer
The 420 is a catalytic converter code, this can be caused by driving while the engine is misfiring, there are usually other codes at the same time like misfire codes or oxygen sensor codes as well, so before replacing an expensive cat converter the drivability issues should be fixed.
It sounds like it may be an ignition circuit problem, possibly spark plugs or ignition coil, is this a 4 cylinder or V6 engine and has anything been done to the engine recently already to try to fix this?