I have posted on this subject before, and have read, and learned it is caused, at least on 12's, when you take your foot off the go pedal, then set the criuse, causing the speed control to search where abouts it was. You can easily reset this code, it takes time, but if you set the cruise, then turn off, turn back on, set cruise, turn off, 25 times. I know this sounds crazy, but you know what?? it works. mine used to shoot up about 10 mph over the speed I wanted to go, and now it is right dead on the money! Y'all 12's try this, and it may work for the over valved folks as well.
-Chris-