Welcome to the forum!
When's the last time the fuel filter was changed? How about fuel injector cleaner, have you used that regularly? If not, you may want to start with the injector cleaner.
While you're letting the injector cleaner do its job, find a fuel pressure guage and check the fuel pressure at the fuel rail. If it's low, replace the fuel filter if that hasn't been done recently. So far all you've done are routine maintenance items that should be done regularly anyway, so no waste of money if they have no effect.
If neither of the above items have an effect, swing by your local automotive parts store and have them check for codes. Your OBD-II car could have pending codes, even though you have not noticed a check engine light.
If there are no codes to point you in a direction, then try turning on the A/C while the engine is running. Does the idle speed drop significantly and stay low? A momentary drop (or increase) followed by a settling out to an engine speed similar to when the A/C is off is good. However, if the engine speed settles at a lower engine speed, then you may want to pull the Idle Air Control (IAC) and clean it and the associated passages.
If you get the issue fixed (even if it has nothing to do with my suggestions
) please post an update to help others out who may experience the same issue.
-Rod