Sounds like there is a fuel trim issue. At idle and part-throttle, the ECU will use the O2 sensors to fine-tune fueling to stay pretty tight at stoich AFR. (~14.7:1). There is a control range of something like +/- 15%, some even as high as +/- 25%. With a scan tool you could watch O2 sensor voltage (or lambda) it should be toggling at idle or reporting lambda = 1. You can see this in my cruise data above (it is basically going slightly rich, slightly lean) if voltage is stuck high (rich) or lambda < 1 and the fuel trims have not gone (-) to correct, then this is a telltale that the ECU is not happy about something so it is ignoring the O2 sensor, you'll be running off the maps only at this point which is not really optimum.

Here is a good description of the trims (it is BMW specific but this model uses a Bosch ECU with similar operation to the Crossfire)

http://ackthud.com/shawnfogg/mixture.htm