Sometimes idles very fast and sometimes it slows

Hello,

It sounds like the idle air control valve (IAC) might be the issue.
The IAC controls the idle speed of the vehicle, when there is a malfunction within the valve is can impact the idle by having too high or low idle. Sometimes cleaning out the IAC can improve the drivability.
The IAC is located on the throttle body.

Here are some helpful articles:

https://www.2carpros.com/articles/how-a-idle-air-control-valve-works
https://www.2carpros.com/articles/idle-air-control-valve-service
https://www.2carpros.com/articles/how-to-replace-an-idle-speed-control-motor-iac

I have attached diagrams for your reference.

Please let me know if you have any questions.
Thank you.