I just acquired this car a few days ago, and at the time we noticed surging while the car is idle. I did some searching around and found that it could be the EICV (electronic idle control valve), but cancelled that out once I replaced it with a known working one. Anybody have any suggestions? I should also add that the gas mileage has not been its best. I have a another theory that it might be the oxygen sensor(s), but it's only a theory.

Any help is appreciated. Thanks.