Failing Performance

Electrical items are togh to diagnose as they fail gradually and have to fail beyond a certain threshold before they will throw a code.
Sometimes ECUs can still be fooled into reacting as if everything is ok and actually make the problem worse.
The first things that come to mind are the Idle Air Control Valve and the O2 sensor.
Even though the code is not there the reasons it is not throwing a code can help diagnose sometimes.
Let me know the following;
Does the car have performance or power problems at speed or anything other than idle?
What did the spark plugs look like when you pulled them?
If thewy were anything other than a Tan/Red, let me know.
I will check fo ryour response and respond ASAP.

The plugs are either black from oil or the fuel/air ratio is rich.
If the exhaust from your car is smokey, smells like burnt oil or sweet like anti-freeze, or if the exhaust puffs smoke when the accelerator is pressed let off and pressed again, (check this by looking out rear view mirror and push the gas, let up and push all the way to floor in a quick sequence and look for exhaust to puff out smoke). The issue is leaning towards a leaking head gasket or leaking valve seals.

If none of the above end up being true, then the problem is most likely related to Fuel Management System failure. This is a tough diagnostics as electrical and mechanical components can be at fault alone or a few problems mixed in all at once. Electrical parts fail gradually or abruptly, this makes tracking electric stuff tough to do. So, I always start with most likely sources, either test or replace depending on price and if it something that is due to be replaced anyway and by process of elimination, we will get it sorted out.
With that said, my first instinct is that there is a fuel system issue and the O2 sensor is seeing a Lean mixture and telling the ECU to dump more gas than is needed in the engine. So, the O2 sensor could have failed or there os an exhaust leak near the sensor.
Let me know the following and we can start eliminating things;
How many times and how long ago has the ECU thrown the O2 sensor code?
Has the O2 sensor been replaced? If so, when?
Open the hood and listen for exhaust leaks or vacuum leaks. Exhaust leaks sound like a tapping valve sometimes or unbaffled exhaust and vacuum leaks. Well they suck.
Let me know that and I will keep and eye out for your reply and get back ASAP.