When i first start my car it idles at like 1800-2000 rpms, and usually after a while it goes down to maybe 1200. but my question is what the normal idle speed is. Also, the rev limiter is what you use to adjust it right??

another thing is when im driving in usually 4th gear and let off the gas and coast, sometimes the engine rpms bounce up and down from like 1000rpms to 1500 back and forth without me giving it any throttle... is that normal or whats the problem there? thanks for the help guys