Recently when I have been coming to a stop my idle bounces up and down between 750 and 1250 RPMs. When I let my foot off the brake it surges higher and bounces up and down between 1250 and 1750 RPMs. This only happens when my truck is at normal operating temperature, and when it is happening I can detect a burnt coolant smell. I have checked my coolant levels and they are fine. This only happens about half the time I drive the truck, but always happens on longer trips. Any ideas as to what may be causing this?