65,000mi. Vehicle stalled on freeway at 60 mph. Coasted to the side of the road, turned ignition off, then on, and continued with no repeat of issue. No OEM code generated.

About 100 miles later, the Service Engine Soon light illuminated with the OEM P0420 code. Vehicle has had no lack of power, increased fuel consumption, stalling, rough running engine, or any other issue that makes me believe it is the catalytic converter (as suggested by the P0420 code). I have driven the vehicle for 600 miles since the initial stall with no other negative mechanical symptoms. Some research suggests possible O2 sensor issues may cause the P0420 code.

Any ideas or suggestions would be appreciated.

 

Not an L car specialist but

front 02 sensor would be first guess here, though usually there is some performance impact with these going bad.

 

Usually when the converter clogs, you will notice a decrease in power which will be more noticeable at higher rpms.. This will be accompanied by a hissing sound (exhaust leaking in front of the converter at the spring loaded flex joint). If you turn the car off at this point, and then restart, the problem will subside a little, but it will return shortly, just as soon as the little metal flakes start restricting the flow through the converter again. O2 sensors are cheaper than converters

 

I considered his cat being clogged but 60K seems low
I suppose anything's possible

 

Derf, I was just listing the symptoms of a clogged cat-which his car doesn't exhibit, so I agreed with your prognosis. My daughter had a clogged converter on her L200 and the problem only got worse quickly. She had a little over 100k on the car so maybe it was due. However, I have never had a clogged converter on a S series car and many were in the 200k+ range.