O2 sensor showed up, and I spent a grand total of 15 minutes changing it out. It took more time to open the hood, grab the tools I needed, and set my drink down without spilling than it did to actually change it out.

Once I did though, the idle smoothed out, and it ran better than it's ever run since I've owned it.

While I was there, I discovered that the engine oil was substantially low - like a quart low off of the 'Add' mark - I guess it's using a little bit of oil, since it's not leaking anymore. I do have a good bit of valve-clatter (kinda sounds like a VW turbo-diesel on hot days, actually), which could mean that I didn't get the valves nailed down as tightly as they need to be... which could be allowing a little bit of oil to sneak by as well. Regardless, the low oil is probably the reason for the Check Engine Light. I guess I'll just have to keep an eye on it a little more closely.

That won't be as much of a problem thanks to the Auto Hobby Shop coming back open hopefully by the middle or end of September.