I need help with a 1998 mercury mystique and the varable cam timing?

car has the ford four cyl. engine with the z tec motor. i have replaced the timing belt 3 times the last time i replaced all the tensioners and ideler pullys. every time the belt just walks right off and then runs into the covers and destroys the belt. ford says the cam with the variable timing is bad and the sensors that supply it with oil need to be replaced. 1000.00 just for parts....anyone know anything about these thanks paul