[Blue Bowtie]

Welcome Aboard!

Yes, I too have experienced the mystery idle/revving. You might want to check the PCV valve and hose, and unless the brakes are HydroBoost, check the brake power booster for leaks. A PCV valve can randomly stick and admit more air at idle than it is designed to pass. This will increase RPM like any other vacuum leak. Internal leaks in a brake power booster can do the same thing.

Another random possibility is the air conditioner. If the AC pressure switch is faulty or if the AC compressor is turned on by the PCM there are also IAC steps added to raise the idle to compensate. I have seen vehicles with failed AC systems have a target idle which hunts up and down.

You might also consider the EGR valve as a potential internal vacuum leak.