*could be a hanging EGR valve. Should be closed until around 2500 RPMs, if its letting any gases by prior to then the ECU cant compensate for it and the result is similar to a vaccum leak. Could always check that out by temporarily blocking the pipe to the back of the intake plenum. If it runs fine and idles strong while blocked off then you have found your problem. If thats not it, then you got a whole big can of worms you get to look forward to testing and checking.
Bad O2 sensor will usually drop fuel mileage considerably.
~Darin <img src="/forums/images/graemlins/baby.gif" alt="" />