My Kia Sportage is accelerating on its own

I have a 1998 Kia Sportage, manual transmission, gasoline engine. About a week ago, it started revving on its own when it's in neutral and when I shift gears (it revs up to 1800 RPM). I changed the spark plugs, air filter, and fuel filter, but there was no change; it's still revving on its own. This is annoying while driving and causes me to use more fuel because I have to wait in long traffic to get home and constantly stop and shift gears.
I hope you can help me solve this problem. Thanks.

I recommend you do that, or have the stepper motor cleaned step by step; carbon buildup causes similar problems... or replace it.

I'm going to follow your advice to clean the stepper motor and see if it works, then I'll let you know how it went.

Check the vacuum hoses that run from the intake gallery to the carbon filter; sometimes a leak or breakage causes the engine to rev up. Check the throttle cable adjustment