Why does my car randomly shut off while driving? As if I just turned the key off?

It has happened a few times now, and is recent. Never have had this issue before. I was driving and suddenly the car just shut off. I pulled off to the side of the road, and tried starting it again, nothing. No fire. I couldn't tell if it was lacking fuel or spark. Check engine light has been on with no real driving issues, got the code reader out and it said I need a new rear o2 sensor. I bought the car with out it and its driven me all over the place till now. I take good care of my eclipse I dont beat on it. One thing I have noticed was the idle is a little bit high sits at about 1100. One one freak event I watched the idle bounce but it hasnt done that since. Please please help me at least put me in the right direction.

Thanks!
chris