For quite some time, my 92 ES300 idle speed has been fluctuating. It will idle at 1200 RPM, then drop to 600 for 30 seconds then surge back up to 1200. It is not linked to air conditioner use since it occurs with the A/C off or on. Other than the fluctuation, engine runs smoothly. Any ideas?

 

Do a throttle body cleaning, which'll most likely fix it.
let me know

 

Put a new throttle body gasket also and tighten to spec when done.