CEL came on with a lumpy idle - what is the culprit?

[mikstew]

I have just returned home from my 1200km trip from Vancouver to Calgary, but about half way home, the dreaded CEL came on. There was no obvious changes with the check engine light coming on, but one thing I noticed was that when I came down from highway speeds, the car had a bit of a tough time keeping a steady idle. It would blob up and down perhaps between 500 and 900, intermittently.

I do know that the car did not do this before the trip. I do not know if it has anything to do with the check engine light.

Before I take my car to the dealership to have them diagnose it, I'd love to hear from anyone who wants to take a stab at what the problem might be.

One thing I thought it could be was that one of the o2 sensors is getting to the failure point, and with all the highway driving I Was doing, it just gave a bad reading for a short time, thus triggering the CEL. Does the emissions CEL automatically go out if it corrects itself (Like for BMW), or once on, does it stay on until cleared? Of course this would not be related to the idle issue.

Another thought I had was that some vac line somewhere has become crusty and cracked.

Any other thoughts are welcome!