Something is making the idle speed surge to fast back to normal?

Hello,

It sounds like an issue with the idle air control valve. Toyota calls it idle up valve.
This valve controls the engine idle speed. It allows more air into the engine when the A/C gets turned on to keep with the extra load.
Over time the valve can become dirty and have air flow restriction, this can impact the idling quality of the engine.
Sometimes cleaning that valve can improve the idle.

Here are couple of helpful articles:

https://www.2carpros.com/articles/how-a-idle-air-control-valve-works
https://www.2carpros.com/articles/idle-air-control-valve-service

I attached diagrams for your reference.

Please let me know of any questions.
Thank you.