I have a 2005 Saturn Ion Redline. Whenever I am coming to a stop I put it in neutral when I get to about 15-20mph, and whenever I am rolling into it the cars idle goes up and down by a good amount every second or so. It also happens in reverse. The car idles just fine when I get to a stop. What could the possible issues be?
This can be caused by a dirty throttle body, even slightly dirty. The throttle body needs to be hospital-room clean. If you do your own repairs, clean out the throttle body with a throttle body spray cleaner and a clean, new, lint-free cloth. Forget about fuel additives -- you need to get in there and wipe it clean, including all sides of all the moving parts and all the surfaces.