I had the same issue. I checked on another forum (forget which one) and also had the codes read. Here is the input I got (disclaimer - I am NOT a mechanic):
Water or debris in the gas can cause this. As gas prices went up ALL of my family's vehicles' mileage went down so I think the gas companies are using extenders in the gas. I switched to premium and the problem pretty much went away.
Since the CEL came on again a couple of more times, I took it to a mechanic who said there was a "soft" code for the o2 sensor in memory. He said that soft codes don't always show up on the meter, but because they are in the system, they can trigger the CEL light at random until they are cleared even if nothing is currently wrong.
So here is my lame advice: Switch to premium (permanently), run a couple of tanks through, then get the codes cleared.
I always vote for trying the easy things first.
For what it's worth,
Phoebe
