Cruise Control Performance Problem

What is your question?

I have a 2010 Mercury Milan Premier (4 cylinders). Since September 2014, I have experienced inconsistent cruise control performance: sometimes it works, sometimes it does not work. This problem first appeared when driving in higher altitudes (1500+ feet). Now, the problem also appears at the lower altitudes (300 feet). Maybe humidity contributes to the problem. The cruiser control turns on/off okay using the on/off button. However, when the car is in motion, the cruise control does not always engage using the -/+ buttons. I know that when the car is stopped, the cruise control does not engage using the -/+ buttons; this makes sense because the cruise control can not set a speed. It is like the cruise control does not always know when the car is in motion. Has anybody else experienced this problem? If yes, do you know the cause/resolution of the problem?