[Showdown]

My guess is that you have both an exhaust leak and a mediocre AFR sensor.

If you have an exhaust leak on the driver's side SSI, then any reading you take for AFR will be affected by that leak. At High RPM you won't see that variation in AFR but at idle and low RPM driving (where the individual exhaust pulses are more pronounced and not smushed together) you'll see a spike or jump in the AFR number as the gas is exiting the leak and not making it to the sensor.