With the vehicle running, take a can of carb cleaner and spray it around your vacuum hoses. If the idle changes (usually goes up) thats a good place to look. DO NOT spray it around the air intake as it will send you on a wild goose hunt looking for leaks that do not exist. Again the idle will usually go up but it can cause it to drop. Either way, there is an issue at that spot. Cracked, loose end, hard and not sealing, you get the point.