Cruise control works (More or less) Why?
 

My brothers '95 dodge dually has a problem with it's cruise. It starts gaining speed without you doing it. It did this since we bought the truck in 2009. At first, we thought it was because where we drove had a bunch of long but mild hills. But since we moved to west Texas (Which is a VERY FLAT), it will still do it. For example, if you set the cruise on a perfectly flat hiway at 60, it will hit almost 80 in less than 3 miles if you do not turn the cruise off. What is causing this?