RPM when starting the truck they go up then come back down?

Hello,

It sounds like you might have an issue with the idle air control motor.
As the IAC gets old it becomes dirty and it can trigger idling issue when there are enough buildup inside.
With a dirty IAC you can get rough idles, it can even prevent the vehicle from idling all together.
Some times you can simply clean out the IAC and the issue goes away.

The Idle Air Control is mounted to the throttle body (see diagram below).

You might find these articles helpful:

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 of any questions.

Thank you.