Hello, I'm Chris.
I will do my best to answer your question fast and accurate using the info you have provided.

You will need a scan tool that can monitor inputs in the computer to watch your fuel trims. Fuel trims are a reading of how much fuel the engine is adding or taking away by way of adjusting the time the fuel injectors stay open.

Check the long term fuel trim with the engine at operating temp. Plus or minus 10% is normal. If the system is running lean, the trim will show positive numbers, over 10% meaning there is a issue now. If the fuel trim is under 10% or close to 0 %, the problem is intermittent which means the engine is running lean at times or the O2 sensor is sticking and indicating a lean.

Attempt to duplicate and monitor the O2 sensor voltage real close to see if the sensor stops switching at times meaning the sensor is bad. Look for vacuum leaks by idling the engine and spraying short burst of flammable carb cleaner around the intake tube and intake gaskets and all PCV and vacuum hoses. If you hit a leak the RPMs will increase. Also check for air getting pulled into the exhaust prior to the O2 from a common exhaust manifold leak from cracks.