I set cruse control and going down hills it speeds up and it does not shift to slow down,have to use my brakes

What could I check to not have to worry about applying my brakes when cruse control is set? I have a 1999 ford taurus wagon.I set cruse control and going down hills it speeds up and it does not shift to slow down,have to use my brakes
it sounds like there is a problem with the actual cruise control mechanism...but i'm not sure how you check that, other than going to a mechanic.