Well, my car did not misbehave even for a second all this weekend. It ran perfectly, so I could not test anything. O2 warning light came up fine, went to fast idle, came down to 800-ish rpm range and pretty much stayed there for 2 days (except for occasional trips to 1,000 rpm right after I disengaged gear into P or N).

I wonder if my problem had anything with a bad gas as car still had some of 2-year old gas in it (mixed with fresh gas). I also plan to replace fuel filter next change I get.

I plan to tackle this problem again as soon as it comes back. Since I am in Chicago area, it is getting close to a winter storage time. If I cannot report back to you guys for a while, I apologize in advance. I will close this thread one way or another - though it may take little longer than I planned. Meanwhile, thank you all.