My car has now shut off twice without warning. Yesterday it happened [at night] while my lights and stereo were on. The car simply shut off. My lights went out for a split second but came right back on. I put it in park, turned the key, and it started up without a problem.
Today the same thing happened after coming to a stop.
So this has occured while in drive and at a complete stop. My RPMs never dropped below 800 until it died.
Now for a slight history:
About a month ago I got a CEL after a hard acceleration and pulled the infamous code 0420. I deleted it after researching what it was. Could this have anything to do with this? I don't see how it could, but who knows? (I still need to fix this

)
Maybe it's my alternator? Supposedly the belt squeals in the morning when warming up if it's funked (please confirm)... but if that's the case then it's muffled by my TH fixs yell for help. Tomorrow I'll check on that before work.
If it's the UIM, I've cleaned it about 6 months ago with B12. I'd do it again but I don't want to change the oil afterwards (YES it does make it dirty!).
Maybe the PCV Valve?
So now my car is sitting in the driveway with the negative battery terminal disconnected to reset the ECU.
Any ideas?