Repair - Used :: 1999 Mazda Idle High While Driving

I have a 99' Mazda B2500 5 speed manual transmission that will idle high sometimes while Im driving, that is in motion coating or in gear. It idles high enough it will cause the truck to speed up or maintain speed when i let off the gas in gear, and when i put it in neutral it will either stay at the rpm it was running at or it will speed up to 2500 - 3000 rpms until i slow to almost a complete stop, then it magically goes away. It will only stop the high idle when i come to a complete stop, even though it will go away just before the truck comes to a stop, if i touch the gas or put it in gear for a California roll it will kick back up to 2500 rpm and keep it up. It does not do this all the time. some times i go months without it happening. Sometimes it goes away in a few minutes and some time it will do it all day on a 18 hour ride. It is totally up to change whether or not its going to act up. Iv had it like this for year and cannot find a pattern to it at all. The amount of gas it gets at high idle speed is not much and is only noticeable on the highway when I go to coast or change gears, in town I notice it every time i come to a stop.