Cruise control

2002 Ford Explorer Started having trouble with cruise control. When you try to set it it wont kick in the green light flashes. How ever if your going down the road and hit resume it will go up to 78 mph and that is the last set speed it was on when it quit working. It will go for differnt lenghts of time and kicks out it may have some thing to do with with starting up a grade that makes it kick out not sure. Even after shuting off the engine and restart it still will run up to 78 if you hit fesume any ideas?

Steve.