Tachometer revs up and down, but when the lights are on works okay

Hello,

Is your check engine light illuminated? If the issue is with the computer, then it will trigger the check engine light. If so, you will need to scan and retrieve the code(s).

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

There could be a number of reasons why the idle is rough.

It could be due to a vacuum leak, worn spark plugs, dirty mass air flow sensor, dirty fuel filter. It can also happen due to a cracked intake boot.

Take a look at this article, it covers the typical issues:

https://www.2carpros.com/articles/engine-hesitates

Got through this guide and let me know what you find or if you have any questions.
Thank you.