Usually this is a vacuum leak somewhere. I've been fighting this code for a while now too, and my car drives fine as well. Replaced my upstream 02 (was old as whatnot, so figured it was time anyways), and still have the code. No vac leaks that I can find. I've even pulled the EGR tube and found it to be airtight, and sealed both ends. I think I'm about to have it smoked to see if I have a leak somewhere that I can't find. Tired of the damn CEL.